Electrocatalytic oxidation of biomass-derived furans to 2,5-furandicarboxylic acid – a review

Abstract

Electrocatalytic conversion of biomass will become necessary for achieving the sustainable production of many kinds of chemicals. In this review, an analysis of catalysts, cell design, coupling reactions, separation and production methods is given for the conversion of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) via electrochemical oxidation. Transition metal-based catalysts (e.g. Ni) can achieve a balance between activity and selectivity. However, issues such as low stability and insufficient active sites hinder the electrocatalytic efficiency of the conversion of HMF into FDCA with transition metal-based catalysts. Enhancements can be achieved through controlling the catalyst morphology and particle size, doping with heteroatoms (N, S, P), crystal structure regulation, and defect/vacancy creation. The design of the electrolytic cell is critical to the stability of the electrocatalytic oxidation of HMF, and the membrane electrolytic cell (MEA) combined with feed separation can reduce side-product formation (e.g. <10% humins) at high HMF concentrations (100 mM) in alkaline electrolytes (1 M KOH). Coupling the electrocatalytic oxidation of HMF with reduction reactions (e.g. hydrogen evolution reaction, nitrogen reduction reaction) can achieve up to twice the energy efficiency, while bifunctional electrocatalysts can balance the potentials and electrocatalytic rates of the cathode and anode. Using organic solvents (e.g. methanol or isobutanol) and controlled temperatures (393 K to 413 K), FDCA (>90%) can be effectively separated and purified at production rates of up to an estimated 33 000 tonnes of FDCA per year (95% HMF conversion) from a feed stream in which KOH can be separated from HMF with current technology. The stability of electrolytic systems under high current densities (>500 mA cm⁻²) is an important factor in industrial applications. Increasing the electrode area or modifying the coordination bonds of the sites can help to remove bottleneck issues associated with long operating hours at high current densities.

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Bingkun Chen

Bingkun Chen is currently pursuing a master's degree in the group of Prof. Haixin Guo, Agro-Environmental Protection Institute, Ministry of Agriculture and Rural Affairs (Tianjin, China). He is currently engaged in research on the electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandiformic acid.

Qidong Hou

Qidong Hou is an associate professor at Nankai University, Tianjin, China. He obtained his bachelor's degree and master's degree in 2013 and 2018, respectively, from the College of Environmental Science and Engineering, Nankai University, China. After graduation, he worked as Director of the Biorefinery Center in the National & Local Joint Engineering Research Center of Biomass Resource Utilization, China. He has coauthored more than 50 scientific articles, 5 books and 20 authorized patents. His research focuses on efficient, economical and environmentally friendly catalytic conversion of biomass into important platform chemicals and their upgrading toward fuels, fine chemicals and degradable plastics to promote the establishment of a more sustainable industry.

Richard Lee Smith Jr

Richard Lee Smith, Jr. is a Professor in the Graduate School of Environmental Studies (GSES)/GP-RSS, Tohoku University, Japan. He obtained his Ph.D. in Chemical Engineering from the Georgia Institute of Technology (USA) in 1985 (Amyn S. Teja, Supervisor). His research focuses on developing green chemical processes with solvothermal and hydrothermal methods. He has published more than 300 scientific papers and is the author of Introduction to Supercritical Fluids published by Elsevier in 2013 and Co-editor of the book series Biofuels and Biorefineries (Zhen Fang, Editor-in-Chief) published by Springer Nature. Professor Smith is Asia Regional Editor for the Journal of Supercritical Fluids.

Xinhua Qi

Dr Xinhua Qi is a distinguished professor of the College of Environmental Science and Engineering at Nankai University. Prof. Qi received his B.S. in environmental chemistry and Ph.D. in environmental science from Nankai University in 1998 and 2003, respectively. His research interest mainly focuses on green processes for biomass conversion into value-added materials and chemicals. He has published more than 160 peer-reviewed scientific papers, and these papers have been cited over 7500 times with an H index of 49. He also has co-authored over 20 patents and 5 books on environmental engineering and biomass resource utilization. Prof. Qi has been selected as a leading talent in the National Ten Thousand Talents Plan and named as one of Elsevier's 2024 “Highly Cited Chinese Researchers”.

Haixin Guo

Haixin Guo is a Professor at the Agro-Environmental Protection Institute, Ministry of Agriculture and Rural Affairs (Tianjin, China). She obtained her M.S. degree in Environmental Engineering from Nankai University in China and her Ph.D. in Environmental Science from Tohoku University. She has published more than 90 peer-reviewed scientific papers, 4 books and 10 authorized patents. She has research interests in sustainable catalyst development, hydrothermal reactions including carbonization, solvothermal and mechanochemical pretreatment of biomass, and the conversion of biomass into chemicals and biofuels.

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Green foundation

1. Electrocatalytic oxidation of biomass-derived furans to 2,5-furandicarboxylic acid (FDCA) offers advantages such as mild conditions, minimal by-products, no organic solvents or additional oxidants.

2. Compared to traditional thermal catalysis, the electrocatalytic process is more energy-efficient and environmentally friendly. The electrooxidation of biomass-derived furans can be coupled with other sustainable reactions, such as carbon dioxide reduction or hydrogen production from water, further enhancing its overall sustainability and resource utilization.

3. Future research will focus on developing more efficient electrocatalysts and innovative reactor designs, such as coupled reactors.

1. Introduction

UN sustainable development goals and strict environmental protection initiatives have motivated researchers worldwide to develop new clean processes based on renewable energy resources.¹ Given its abundant availability and renewability, biomass presents a promising alternative to fossil fuels for the production of chemicals and biofuels.² Global awareness of the environment makes it imperative to develop sustainable chemical processes for producing platform chemicals from biomass.^3–5

Selective catalytic transformations of lignocellulosic biomass into platform chemicals need to be energy-efficient, economical and sustainable.^6–9 As some of the most promising biomass-derived compounds, furans such as furfural (FUR)¹⁰ or 5-hydroxymethylfurfural (HMF)^11,12 can be converted into fuels or commodity chemicals by hydrogenation, etherification, or oxidation.^13–15 Oxidation of furfural or 5-hydroxymethylfurfual to furoic acid, maleic anhydride (MA) or 2,5-furandicarboxylic acid (FDCA) are key intermediates in biorefineries that will produce fuels, polymeric materials, pesticides or pharmaceuticals (Fig. 1a).^16–20 Among these chemical products, FDCA is expected to enable the production of green polymers such as polyethylene-2,5-furan dicarboxylic acid (PEF). PEF is expected to replace petroleum-based terephthalic acid (TPA), which is currently used in the production of poly(ethylene terephthalate).

Fig. 1 (a) Lignocellulosic electrocatalytic oxidation upgrading to value-added chemicals via furfural (FUR) and 5-hydroxymethylfurfural (HMF) platform molecules; (b) classification of electrocatalytic systems for furan-EOR.

Thermal catalysis,^21–23 photochemical catalysis,^24–26 biocatalysis^27,28 and electrocatalysis^29–32 are generally applicable to the oxidation of biomass. Thermal catalytic systems tend to use strong oxidants such as chromate or KMnO₄, which must be eliminated because these chemicals have properties of concern.³³ Thermal catalytic systems have low reaction selectivity and they produce a large amount of hazardous wastewater.^34,35 Photocatalytic systems have many favorable attributes, although they have high requirements for light conditions and have limited application to solutions containing impurities.^36,37 Biocatalytic systems have high selectivity; however, their product solutions require intensive separation methods, their productivity is sensitive to solution conditions and their conversions generally require long reaction times.^38,39 Electrocatalytic oxidation uses an electrochemical potential at an electrode surface to drive electron transport, which reduces the reaction time and energy required to achieve conversion, thereby improving overall energy efficiency.⁴⁰ Because no chemical oxidants are required, the oxidation efficiency of an electrocatalytic system can be improved and the environmental burden can be lowered, because the processes typically operate at normal temperature and pressure.^41,42 The production of FDCA electrocatalytic oxidation is possible with metal or non-metal-based catalysts in an electrolyte.^43,44

The activity of metal-supported catalysts is influenced by factors such as the type and structure of the support, as well as the catalyst morphology, atomic composition of the metal and size of the metal particles.^45–47 Current research focuses on the design of high-efficiency catalytic electrodes, the elucidation of reaction mechanisms and the design of electrolysis reactors. Scaleup methods are under development for the industrial application of electrocatalytic reactor systems.^48,49 Economic aspects such as the price of raw materials and the performance of industrial equipment affects the development of practical strategies based on oxidation efficiency.^50,51 Currently, there is much research activity on improving the economics of industrial production through the choice of biomass-derived substrates with a focus on platform chemicals by coupling oxidation and reduction reactions.^52,53

Although several reviews^54,55 have considered the catalytic properties of HMF oxidation, there is a lack of specific analysis of the designs, structures and possible reaction mechanisms of electrocatalysts, while product separation, cell design, paired electrolysis engineering, and industrial applications have received less attention when considering the overall conversion and separation processes. This work aims to fill this gap through analysis of the design of electrocatalysts for HMF oxidation and by examination of possible processes for HMF oxidation. The review considers recent breakthroughs in catalysis for the electrochemical oxidation of biomass-derived furan (HMF) with discussions of reaction mechanisms, reactor types, and product separation (Fig. 1b). Techno-economic assessment for large-scale industrial application is performed as a method to stimulate research and development of electrocatalytic oxidation systems.

2. Electrocatalyst design for furan oxidation

Many materials have been evaluated for fabricating electrocatalysts that promote the oxidation of biomass-derived compounds, including noble metal-based electrodes (e.g. Pt, Pd, Au)^56–61 and transition metal based electrodes (e.g. Ni, Co, Fe)^62,63 loaded with hydroxide,^64,65 phosphide,⁶⁶ sulfide,^67,68 nitride,^69,70 or non-metal alternatives (e.g. carbon and 2,2,6,6-tetramethylpiperidinyl-1-oxide (TEMPO)).^71,72 The most commonly used electrode supports (carriers) include nickel foam, carbon cloth, titanium foil, stainless steel, iron foam, cobalt foam and copper foam.^73–75 A flow chart of electrocatalyst preparation using nickel foam as a support is shown in Fig. 2.

Fig. 2 Basic steps (pretreatment, element impregnation, calcination) required for electrocatalyst preparation with nickel foam as a support (carrier).

2.1. Noble metal electrocatalysts

Similar to heterogeneous catalysis, electrocatalysis involves a reaction at an active site of a solid material; however, transformation occurs at the electrode–electrolyte interface of the electrocatalyst. Noble metal electrocatalysts have been widely studied for the electrocatalytic oxidation of biomass-derived furans^76,77 due to their high activity, which has been established in the field of heterogeneous catalysis.

2.1.1. Monometallic electrocatalysts. Noble metals based on Pt, Au, Ru, and Pd metals are highly active for electrocatalytic oxidations.^78–80 Vuyyuru and Strasser⁸¹ applied Pt foil electrodes for the electrocatalytic oxidation of HMF; however, at a current density of 0.44 mA cm⁻², only 18% DFF oxidation intermediate was obtained and practically no FDCA was formed. Roman et al.⁸² suggested that the selectivity and activity of electrodes used in furfural oxidation could be optimized to form furoic acid (FA) by limiting the coordination of furan on the Pt electrode surface. Furthermore, furoic acid can be used as a raw material for carboxylation to produce FDCA, with little pretreatment, avoiding separation steps. Cao et al.⁸³ used Pt and Ru as catalysts to assemble membrane electrodes and compared the catalytic properties of electrode materials for HMF production, where it was found that Pt and Ru monometallic electrocatalysts achieved HMF conversions of only 12% and 5% after reacting for 17 h, respectively, which was attributed to the competitive adsorption of HMF carbonyl-containing substances onto active sites. Parpot et al.⁸⁴ converted 56% furfural into furoic acid by electrolysis on a gold electrode for 13 h and obtained 95% selectivity. In view of the limited electrochemical performance and high cost of single noble metal-based electrode materials, strategies have been developed to adjust their atomic structure by doping with heteroatoms, especially non-metallic doping (such as carbon, boron, sulfur and phosphorus) to promote the intrinsic activity of electrode materials.^85,86 Chadderdon et al.⁸⁷ prepared noble metal Pd and Au nanoparticle catalysts using carbon black as a support. At a current density of 3–7 mA cm⁻², the selectivity of Au/C to the intermediate product HMFCA was as high as 98%, but the catalyst was unable to promote further oxidation of HMFCA to the final product FDCA. Pt/C is able to achieve 29% FDCA yield at a potential of 0.9 V vs. RHE, which is a higher yield and a lower potential than that of pure Pt electrodes (entry 1, Table 1). Xu et al.⁸⁸ used a bifunctional electrocatalyst based on ruthenium polyethyleneimine (Ru(III)–PEI)-supported carboxyl-modified carbon nanotubes (Ru(III)–PEI@MWCNTs), to achieve 94% FDCA yield and 94% FE (faradaic efficiency) after reacting for 27 h using HMF as a substrate. As oxidation proceeded, the electrochemical reactor could also catalyze nitrate reduction in cathode-coupled electrocatalysis (entry 2, Table 1).

Table 1 Oxidation performance of noble metal electrocatalysts. Electrolyte is 1.0 M KOH unless otherwise noted

Entry	Electrocatalyst	Initial HMF (mM)	Current density (mA cm^−2)	Potential (V vs. RHE)	Conv. (%)	Yield (%)	FE (%)	Ref.
a 0.1 M KOH.
1^a	Pt/C	20	—	0.9	97	29	—	87
2	Ru(III)–PEI@MWCNTs	1	0.5	1.34	—	94	94	88
3^a	Pd1Au2/C	20	—	0.9	98	83	—	87
4	AuPd–LbL	10	3.09	1.43	23.6	17.5	85.8	60
5	Pt/NiSx	10	10	1.47	100	100	98	92
6	RuNPs–Ni(OH)2/NF	10	40	1.35	99.4	99.4	92.2	93
7^a	PtRhPdIrRuAu	1	—	1.69	—	63.4	—	98

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2.1.2. Bis/polymetallic electrocatalysts. Single-metal electrodes typically suffer from low current density, poor yields and low stability.^89,90 To improve catalyst stability and catalytic performance, bimetallic or polymetallic electrodes are found to be beneficial for selective electrooxidation of furans.⁹¹ Chadderdon et al.⁸⁷ designed Pd–Au bimetallic electrodes (Pd₁Au₂/C) that could complete the entire oxidation process at a lower potential than that of a single metal, and achieved 83% FDCA yield in 1 h electrolysis time; this performance was superior to that of single metal electrodes (entry 3, Table 1). Cao et al.⁸³ examined Pt–Ru bimetallic electrodes at 324.15 K for 100 h to achieve 54% HMF conversion, but only small amounts of the target product, FDCA, were obtained. Thus, metals with appropriate activities for electrocatalytic oxidation need to be selected that will enable completion of the HMF to FDCA oxidation pathway or otherwise, reaction intermediates will remain. According to the catalytic properties of metals, Park et al.⁶⁰ designed a hybrid three-dimensional electrode with Au and Pd nanoparticles (NPs) supported on nano-graphene oxide using layer-by-layer assembly (entry 4, Table 1). The special layered structure promoted rapid oxidation of HMF to HMFCA by the outer Au nanoparticles and further catalyzed the oxidation of HMFCA to FDCA by inner Pd nanoparticles. While the FDCA yield of the three-dimensional composite electrode was low (18%), the work demonstrated the principle of nanostructure design in an electrode. Noble metals can be combined with transition metals to achieve controlled reactivity and selectivity. Wang et al.⁹² prepared a Pt/NiS_x nano-catalyst with carbon black as the support (entry 5, Table 1), which could efficiently convert HMF into FDCA and cleverly assembled the system into a fuel cell, and measured the discharge efficiency of the fuel cell to be as high as 21.2 mW cm⁻². Cheng et al.⁹³ synthesized a Ni (OH)₂ nanosheet array decorated with Ru nanoparticles (RuNP) on nickel foam (NF), which could achieve a current density of 40 mA cm⁻² at a potential of 1.35 V vs. RHE and obtained an FDCA yield of nearly 99.4% (entry 6, Table 1). Ru on Ni(OH)₂ was found to promote surface reconstruction of the catalyst, enhance the electronic interaction between HMF and the catalyst, and thus accelerate the oxidation reaction.

The synthesis of high-entropy alloys (HEAs) is one strategy for improving the catalytic performance of noble metal electrodes. High-entropy alloys are defined as new materials that contain five or more elements, with each element accounting for 5–35% of the total composition.^94,95 Alloying can effectively adjust the adsorption energy of reactants and key intermediates on metal surfaces.⁹⁶ The tunability of high-entropy alloys allows for fine control of the catalytic activity and selectivity, making them very promising candidates for catalytic optimization.⁹⁷ Huang et al.⁹⁸ synthesized nanosized PtRhPdIrRuAu HEA NPs by co-reduction of noble–metal precursors in triethylene glycol, which were used as the catalyst for electrooxidation of HMF and they found that the composition of HEA NPs played an important role in determining the oxidation pathway. The five-membered HEA NPs formed by adding Ru or Au were more inclined to form DFF by direct oxidation of the hydroxyl group of HMF, and further promoted the formation of FDCA. More importantly, the high entropy noble metal alloy was active in both alkaline and acidic media, and had a wide selectivity for products; this provides an interesting way to regulate the oxidation products of HMF (entry 7, Table 1).⁹⁸ To summarize, noble metals have merits of high efficiency, stability, and catalytic performance at low temperatures, but have inherent shortcomings due to high cost, catalyst inactivation, and selectivity that general lower their application. On one hand, noble metals are not cost-effective on an industrial scale for HMF transformation because, from their mechanistic characteristics, the carbonyl group of HMF usually occupies a surface site on the metal through strong adsorption, which easily leads to catalyst deactivation. Although noble metal electrocatalysts can be modified by doping with heteroatoms for low activation potentials, they can generally only be used under low current densities (<10 mA cm⁻²), which limits their application in industrial settings.

2.2. Non-noble metal electrocatalysts

Transition metals are used in electrocatalysis due to their characteristics of being able to fully exert their catalytic activity when driven by an electric potential^99–101 and also because they are cost-effective. Optimizing the transition metal composition and structure is required in electrode design.^102,103 Non-noble metal-based electrodes (e.g. Ni, Co, Fe, Cu) are reviewed from the perspective of catalytic activity and material structures that are important for achieving high performance.

2.2.1. Monometallic electrocatalysts. Nickel is one of the most popular materials for electrodes used in the electrocatalytic oxidation of HMF due to its high catalytic activity for aldehyde and hydroxyl groups.¹⁰⁴ Taitt et al.¹⁰⁵ investigated and compared the intrinsic catalytic properties of MOOH (Ni, Co, and Fe) for oxidation of HMF. They found that NiOOH emerged as the most effective electrochemical catalyst for this reaction. At a potential of 1.47 V vs. RHE, FDCA yields of up to 96% were observed with high faradaic efficiency. Although CoOOH can initiate HMF oxidation at a comparatively low potential, its rate of oxidation is not high enough to support high current densities for continuous and prolonged periods of oxidation. Notably, FeOOH did not demonstrate any catalytic activity for HMF oxidation under an initial water oxidation potential. Nevertheless, trace amounts of FDCA were produced at a potential of 1.71 V vs. RHE, indicating that FeOOH possessed some capability for oxidizing HMF to FDCA, albeit with low efficiency. Grabowski et al.¹⁰⁶ performed an early study on the electrochemical oxidation of HMF with a Ni(OH)₂/NiOOH electrode anode in a strongly alkaline solution (1.0 M NaOH). However, the catalytic activity of mono-metallic nickel was limited by its small electrochemically active area and low number of active sites. The structure of the nickel catalyst can be regulated by means of doping with heteroatoms, surface heat treatment and nano-structure construction, to improve its electrochemical performance.¹⁰⁷ For example, Ni NPs,^108,109 NiO,^110–112 Ni₂P,¹¹³ Ni₃S₂,^114,115 and NiB^116,117 have shown good catalytic activity. Wang et al.¹¹⁸ demonstrated that phase transition kinetics and HMF oxidation activity of nickel nanoparticles could be regulated by creating self-assemblies with different particle aggregate structures. Nanoparticle assemblies with ordered nanoarray structures have high activity (99.8% HMF conversion, 99.2% FDCA yield) at 1.36 V vs. RHE (entry 1, Table 2) (Fig. 3a–c). Liu et al.¹¹⁹ reported that a nickel–phytic acid hybrid (Ni–PA) electrode prepared with natural phytic acid as a building block could efficiently catalyze the oxidation of HMF to 2,5-furandicarboxylic acid (FDCA). Due to the coordination of nickel ions and phosphate groups of phytic acid molecules, high FDCA yields (99.1%) at 1.6 V vs. RHE (entry 2, Table 2) were achieved. Yang et al.¹²⁰ constructed nickel oxide nanosheet structures supported by nickel hydroxide using a hydrochloric acid chemical in situ corrosion method. A 99% yield of FDCA with 96% FE was obtained at 1.47 V vs. RHE and oxidation current of 180 mA cm⁻² (entry 3, Table 2).¹²⁰ You et al.¹¹³ synthesized a 3D bifunctional Ni₂P NPA/NF electrocatalyst on nickel foam by direct phosphorylation to obtain 98% yield of FDCA with 100% FE at 1.42 V vs. RHE, which demonstrated that phosphorylation was able to improve the catalytic performance (entry 4, Table 2). They applied this phosphorylated electrode to the electrocatalytic oxidation of furfural and achieved 98% conversion of furfural and a 94% yield of 2-furoic acid.¹²¹ A bifunctional electrocatalyst on a Ni₂P foam-based nickel array (Ni₂P/Ni/NF) gave FE values approaching 100% for the synthesis of 2-furoic acid and H₂ in alkaline solution.¹²² Barwe et al.¹¹⁷ designed high specific surface area (127.4 m² g⁻¹) nickel boride (Ni_xB) as the electrode in a two-compartment flow cell device to obtain nearly 100% FE and 98.5% FDCA yield. The importance of Ni³⁺ for HMF oxidation was also revealed.¹¹⁷ Namely, the conductivity of the catalyst rapidly dropped when Ni sites became oxidized, which caused a rapid loss in catalytic activity; however, charge transfer could be enhanced by introducing a second nonmetallic component.¹²³ Song et al.¹¹⁶ found that appropriate doping of NiB_x with P could change the electronic structure and distribution of nickel-containing surfaces and could increase nickel hydroxide species on the catalyst surface. Results of AC impedance spectroscopy (EIS) verified that doping with P improved the catalyst's capacity for charge transport (Fig. 3d–g). Gao et al.¹²⁴ designed a core–shell structured catalyst with conductive NiSe nanowires as the core and high-valence NiO_x as the shell, and obtained a high catalytic performance with an FDCA yield of 99%.

Fig. 3 (a) Schematic illustration of Ni/CP preparation; (b) Tafel plots; (c) conversion of HMF, yield of FDCA, and FE. Reproduced from ref. 118 with permission from Elsevier, copyright 2024. (d) Reaction diagram of NiB_x–P_0.07; (e) XPS spectra of Ni 2p; (f) Nyquist plots; (g) electron transport process during the HMFOR. Reproduced from ref. 116 with permission from American Chemical Society, copyright 2020.

Table 2 Electrooxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid with non-noble metal electrodes. Electrolyte is 1.0 M KOH unless otherwise noted

Entry	Type	Electrode	Initial HMF (mM)	Current density (mAcm^−2)	Potential (V vs. RHE)	HMF Conv. (%)	FDCA yield (%)	FE (%)	Ref.
a 0.1 M KOH. b 1.0 M NaOH. c Total faradaic efficiency.
1	Mono-metallic	Ni NPs	5	10	1.36	99.8	99.2	99.5	118
2		Ni–PA	10	10	1.6	100	99.1	90	119
3		Ni(OH)2/NF	10	180	1.47	99.5	99	96	120
4		Ni2P NPA/NF	10	10	1.42	100	98	100	113
5		CoB	10	55	1.45	100	94	100	140
6		Co3O4	10	12	1.04	80	95.7	127^c	142
7		Fe2O3/Nb2O5/C	10	60	0.68	100	96.6	—	152
8^a		NCF	5	4	1.62	100	96.4	95	154
9		CoO-CoSe2	10	10	1.43	100	99	97.9	134
10	Bis/poly-metallic	NiCo–MOF	50	600	1.40	—	99	80	167
11		NiCo–LDH	—	10	1.23	98.6	98	99.4	168
12		Co–NixP@C	10	10	1.56	100	100	98.9	195
13		Ni3S2–MoS2	20	10	1.44	100	96	99	173
14		NiFe LDH	10	10	1.23	98.6	98	99.4	178
15		NixSey–NiFe@NF	10	50	1.37	99.6	99.3	96.7	182
16		CuNi(OH)2	5	9.2	1.45	100	93.3	—	185
17		CuMn2O4	10	20	1.33	100	98.4	96	162
18^b		NiCoMn–LDHs	1	10	1.42	100	91.7	90	169
19		CuxS@NiCo–LDH	10	10	1.34	100	∼99	95	187
20		NiVCo–LDH	10	10	1.37	—	99.7	97.8	194

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The electrical conductivity and catalytic stability can also be improved by coating a Ni-based catalyst with carbon in which carbon modulates the electronic state of Ni species and stabilizes the catalyst morphology.¹²⁵ Sang et al.¹²⁶ designed a plate carbon-coated nickel nanocrystalline (Ni@C) electrode. The coupling of Ni nanoparticles in a carbon layer was found to regulate the electronic structure of Ni and afford 100% HMF conversion, 97.7% FDCA yield and 97.4% FE. Carbon-coated porous nanosheet materials (Ni₃N@C) on nickel foam substrates were prepared by hydrothermal and high-temperature ammonia treatment, and interfacial charge transfer between the carbon layer and Ni₃N was observed.¹²⁷ Liu et al.¹²⁸ developed a chelator-assisted electrodeposition strategy that addressed some of the key challenges in catalyst scalability and paired (coupled) electrolysis efficiency. The Ni-0.25EDTA2Na anode had an adjustable two-dimensional layered nanosheet structure and achieved 98.7% FDCA yield and 159.1 g L⁻¹ hydrogen yield. Energy consumption was reduced by 27.5% compared with conventional OER–CO₂ systems and the system provided hundreds of times more economic value through high-value product cogeneration.

Enhancing the dispersion of active components by controlled preparation is another way to improve the catalytic performance of an electrode. Highly distributed metal electrocatalysts can be prepared by direct application of covalent organic frameworks (COFs) or metal organic frameworks (MOFs) as electrodes.^129–131 Cai et al.¹³² achieved 58% FDCA yield using a nickel-modified COF membrane TpBpy–Ni@FTO electrode, and then studied the oxidation of FFCA to FDCA; this is generally considered to be an easy-to-control step. Sekar et al.⁶² prepared a single crystal Ni–MOF with a two-dimensional rod-like form as a catalyst. Incorporation of redox-active riboflavin (Rbf) into the Ni–MOF electrode changed the intrinsic properties of Rbf–Ni–MOF and induced electrical conductivity without changing the crystallinity or other properties, which helped the RBF–NI–MOF electrode to show enhanced electrocatalytic activity (FDCA yield >90%, FE >95%). High-alkali environments were found to be conducive to the formation of Ni³⁺N(OH)ads active species and facilitated the adsorption of HMF on the electrode surface. A high concentration of OH^– in a high-alkaline environment favored the oxidation of Ni (Fig. 4) and promoted the formation of Ni³⁺, so the onset potential of HMFOR in 1 M KOH (1.27 V vs. RHE) was lower than that in 0.1 M KOH (1.35 V vs. RHE).¹³³

Fig. 4 Schematic diagram of the effect of OH^– concentration on HMF oxidation reaction. (HMFCA: 5-hydroxymethyl-2-furancarboxylic acid; DFF: furan-2,5-dicarbaldehyde; FDCA: 2,5-furandicarboxylic acid; FFCA: 5-formyl-2-furancarboxylic acid.) Reproduced from ref. 133 with permission from Elsevier, copyright 2021.

Co-based materials, such as CoX (X = B, Si, P, Te, As)^134–137 and Co₃O₄ ^138,139 have been widely used in the electrocatalytic oxidation of HMF. Weidner et al.¹⁴⁰ used a variety of cobalt–metalloid alloys (CoX) as potential catalysts for HMF oxidation. Those researchers found that CoB exhibited the best catalytic activity with the lowest initial potential of 1.45 V and FDCA yield of 94% among Co-based materials (entry 5, Table 2).¹⁴⁰ Sun et al.¹⁴¹ prepared the N–Co₃O₄/NF electrocatalyst using a controlled nitrogen doping strategy and found that the N-dopant could induce the generation of defects such as oxygen vacancies, jointly regulating the electronic structure of Co and improving the conductivity. Those researchers achieved 96.4% FDCA yield and 99.5% HMF conversion at a potential of 1.38 V vs. RHE at a current density of 50 mA cm⁻².¹⁴¹ Pan et al.¹⁴² developed a self-supporting ultra-thin Co₃O₄ nanoarray electrocatalyst to produce 2,5-dihydroxymethylfuran (DHMF) (78.8%) and FDCA (95.7%) at the cathode and anode respectively, with a total (cathode and anode) FE of 127% (entry 6, Table 2). Huang et al.¹³⁴ found that electrodes showed enhanced electrocatalytic oxidation performance when doped with a small amount of Se compared with CoO. Micro-doping with Se metal increased the content of oxygen defects on the CoO surface of the electrode, resulting in a larger specific surface area and a smaller charge transfer resistance than that of the non-doped material.¹³⁴

Iron, which is an abundant and inexpensive transition metal, might be expected to be directly applicable to electrocatalysis.^143,144 However, Lyons and Brandon¹⁴⁵ found that compared with Ni and Co metals, Fe electrodes had the lowest electrocatalytic performance, and it was necessary to modify them. Anion-substituted iron-based catalysts (iron-based phosphates,¹⁴⁶ sulfides¹⁴⁷ and nitrides¹⁴⁸) can be applied to electrocatalytic oxidation, and the incorporation of anions can effectively regulate the electronic properties of the metal center, thereby affording FDCA yields of up to 98%. Liang et al.¹⁴⁹ prepared a self-supporting array of FeP nanorods using Fe₂O₃ precursor grown on carbon cloth by a hydrothermal reaction, which functioned reliably in either neutral or alkaline solutions. Iron oxide nanoparticles were coated with polydopamine and were then carbonized and phosphorized to create carbon-shell-coated FeP nanoparticles, which only slightly decreased in activity after 5000 CV cycles.¹⁵⁰ Tan et al.¹⁵¹ fabricated FeS/FeO_xH@Fe nanosheets through electrodeposition, employing thiourea and FeSO₄·7H₂O as sources of sulfur and iron, respectively. High electrical conductivity and stability were achieved because of the generation of crystalline and amorphous interfacial structures. Heterogeneous structures with more interfacial regions could shorten the charge transfer distance and time. Yuan et al.¹⁵² constructed a superlattice α-Fe₂O₃/Nb₂O₅/C nanorod photocatalyst. The cross-growth of α-Fe₂O₃ and Nb₂O₅ thin layers greatly increased the phase interface between the two, and effectively improved the catalytic activity of the electrode, which afforded 100% HMF conversion and 96.6% FDCA yield (entry 7, Table 2).

Copper-containing compounds are almost inactive for water oxidation, which is a useful property when oxidizing furans in aqueous solutions.¹⁵³ The available valences of Cu facilitate electron transfer in oxidation processes. Nam et al.¹⁵⁴ achieved 96.4% yield of FDCA with 100% conversion of HMF over a nanocrystalline copper foam (NCF) electrode (entry 8, Table 2). 99% selectivity of FDCA with 97.9% faraday efficiency was achieved over a CoO-CoSe₂ electrode (entry 9, Table 2).¹⁵⁵

Non-noble metal catalysts, such as Mn, Mo, V and Sc, their alloys, and oxide/hydroxides, exhibit electrocatalytic properties.^156–158 Manganese oxide (MnO_x) nanosheets can catalyze the oxidation of HMF in sulfuric acid electrolytes at pH = 1, with FDCA yields of up to 53.8%, but produce 21% maleic acid by-product.¹⁵⁹ Wu et al.¹⁶⁰ prepared mesoporous Sc₂O₃ with a high surface area (146.6 m² g⁻¹) by nano-casting; abundant oxygen vacancies (V_o) regulated the crystal and electronic structure of the electrode and promoted the adsorption and electrooxidation of HMF.

2.2.2. Bis/polymetallic electrocatalysts. Single-metal electrocatalysts have limitations in terms of efficiency and stability. Bis/polymetallic electrocatalysts can exhibit synergistic effects due to the presence of multiple metal atoms, with examples being Ni–Co,¹⁶¹ Cu–Mn¹⁶² and Ni–Fe–Co,^163,164 all of which show advantages for electrochemical oxidation. The use of bis/polymetallic electrocatalysts enables electronic structure optimization.^165,166 Gao et al.⁵⁵ studied the interactions between Ni and Co electrodes by comparing the catalytic activities of NiO, Ni_xCo_3−xO₄, and Co₃O₄ materials. Increasing the Ni content lowered the Tafel slope and enhanced reaction kinetics, while initial potential and charge transfer resistance decreased with an increase in Co content. The most effective NiCo₂O₄ nanowires fabricated could achieve 90% FDCA yields when electrolysis was conducted at 1.55 V vs. RHE constant voltage.⁵⁵ Deng et al.¹⁶⁷ prepared a nanoplate–nanoarray electrocatalyst (nickel–cobalt hydroxyl) by cyclic voltammetry-assisted bimetallic NiCo–MOF nanoarrays. NiCo–MOF achieved a current density of 600 mA cm⁻² with a potential of 1.4 V vs. RHE in a 1.0 M KOH solution for a concentration of 50 mM HMF; this means that it is possibly applicable to large-scale industrial production (entry 10, Table 2) (Fig. 5a–c). Lu et al.¹¹¹ demonstrated that three-dimensional layered nanostructure electrocatalysts (NiO–Co₃O₄) with a high concentration of defect interface sites and cation vacancies could regulate the electronic properties of Co and Ni atoms and provide stability and high catalytic activity.¹¹¹ Schneiderova et al.¹⁶⁸ prepared stacked structure NiCo–LDH three-dimensional porous nanosheets by electrodeposition, and combined the characteristics of cobalt-based high stability and nickel-based high specific capacitance to improve the electrochemical performance of nickel–cobalt electrodes (entry 11, Table 2). Two ternary transition metal electrocatalysts, NiCoFe–LDH¹⁶³ and NiCoMn–LDH,¹⁶⁹ were designed based on NiCo–LDH. Compared with the binary LDH catalyst, the addition of Fe caused NiCoFe–LDH to exhibit a lower overpotential, smaller charge transfer resistance, larger double layer capacitance and better durability than NiCo–LDH. The NiCoFe–LDH electrode gave an HMF conversion of 95.5% and an FDCA yield of 84.9% in 1 h. Ren et al.¹⁷⁰ used CoNiMnMoPd high entropy alloy (HEA) to create an electron-deficient environment that stabilized the high-oxidation state of nickel (Ni³⁺). Unlike the repeated changes in the valence state of nickel in conventional catalysis, HEA ensures that nickel remains in its high oxidation state and dehydrogenation preferentially occurs at adjacent non-nickel sites. This high-entropy environmental strategy introduces a new paradigm for catalyst design, demonstrating how atomic-scale component diversity modulates the electronic structure to enhance adsorption and reaction kinetics, providing a reference for highly stable selective electrocatalysts.

Fig. 5 (a) LSV curves of t-MOFs with various Ni to Co ratios; (b) current densities of t-Ni MOF, t-Co MOF and t-Ni1Co1 MOF at different potentials and electrodes. (c) Tafel slopes derived from LSV. Reproduced from ref. 167 with permission from Elsevier, copyright 2020. (d) Gibbs energies of hydrogen adsorption and (e) adsorption energy of H* on Co–NixP@C and NixP@C. Reproduced from ref. 195 with permission from Elsevier, copyright 2023. (f) Chemisorption models of HMF heterostructures, and MoS₂–NiO heterostructures. Reproduced from ref. 173 with permission from Wiley, copyright 2022.

Nonmetallic N, S, P modification or doping of bis/polymetallic electrocatalysts can produce hybrid effects or promote the formation of heterogeneous interfaces, which can regulate catalytic performance.¹⁷¹ The nanowire-like network hierarchy of carbon-coated cobalt-doped nickel phosphide hybrid catalysts (Co–Ni_xP@C) (entry 12, Table 2) provided abundant active sites, interfacial lattice mismatches, and defects,¹⁷¹ which gave a high electrocatalytic performance for furan oxidation. Cobalt doping can effectively reduce the energy barrier of the constant-speed step, promote the activation and adsorption of HMF molecules, and thus enhance the oxidation activity of HMF (Fig. 5d). The Gibbs energy of hydrogen adsorption shows that Co–NixP@C (−0.67 eV) has a higher energy value (−0.54 eV) than NixP@C, confirming that cobalt doping is favorable for H* adsorption on Co–NixP@C (Fig. 5e). The combination of metal sulfide nanoclusters and two-dimensional MOF nanoarrays expand the electrochemically active surface area and increase the number of active catalytic sites, thereby improving electron transport.¹⁷² A Ni₃S₂–MoS₂ nano-heterojunction bifunctional catalyst exhibited favorable HMFOR and HER properties, as evident from its application, where a current density of 10 mA cm⁻² required a potential of 1.44 V vs. RHE, which was much lower than that for pure water splitting (entry 13, Table 2).¹⁷³ An N-doped carbon supported CoCu bimetallic catalyst was synthesized through a double solvent method using ZIF-8 as a sacrificial template, which provided active sites with a strong molecular oxygen activation capacity.¹⁷⁴

Compared with Ni-based and Co-based electrocatalysts, Fe-based catalysts have poor electrical conductivity and poor oxygenophilicity, and they leach ions more easily into the electrolyte, resulting in a low electrochemical oxidation activity.^175,176 The incorporation of iron into other metals or metal (oxygen) hydroxides, however, is one of the ways to improve the intrinsic activity of Fe-based materials.¹⁷⁷ NiFe–LDH nanosheets prepared by a hydrothermal method with carbon fiber paper as the anode achieved 98% FDCA yield and an FE of 99.4% at a potential of 1.23 V vs. RHE (entry 14, Table 2).¹⁷⁸ Previous studies have confirmed that the higher valence state of Ni or Fe in nickel–iron-based electrocatalysts is beneficial for improving the catalytic performance of the oxygen evolution reaction (OER),¹⁷⁹ so other variable valence transition metal cations can be introduced into the HMF oxidation reaction to change the electronic structure and chemical environment of atoms on the layer. Hao et al.¹⁸⁰ induced the formation of Ni(OH)₂ heterogeneity by adding Fe to adjust the electronic structure of Ni(OH)₂, and with the assistance of Co, which improved the conductivity of Ni(OH)₂ and promoted electron transport (Fig. 5f). Co and Fe co-doped Ni(OH)₂ heterogeneous materials exhibit powerful multifunctional activity in a variety of electrocatalytic oxidation reactions, with a high selectivity of 96.78% and an FE of 92.7%.¹⁸⁰ Since iron is an abundant and inexpensive metal, it is necessary to develop other effective strategies to improve its resistance to electrocatalytic oxidation. Hausmann et al.¹⁸¹ demonstrated the electrocatalysis of HMF to FDCA with FE reaching more than 90%, by using the structural order and high conductivity of intermetallic compounds produced by Sn, Si, and Fe. Zhong et al.¹⁸² constructed a three-dimensional layered core–shell nanostructure (Ni_xSe_y–NiFe@NF (LDH)), which resulted in 99.3% yield of FDCA and 99.7% yield of furfuric acid from HMF and furfural, respectively. The presence of Fe in the catalyst modulated the electronic structure of Ni, exposing rich active sites and facilitating electrolyte diffusion, thus promoting the catalytic activity (entry 15, Table 2).

Adding another metal to Cu(OH)₂ is an effective way to improve the electrooxidation performance of Cu-based electrodes, where HMF conversions close to 100% have been achieved in weakly alkaline electrolytes.^183,184 Chen et al.¹⁸⁵ prepared a Cu–Ni bimetallic electrode with CuNi(OH)₂ structure by an incipient wetness impregnation method. The number of active sites increased with the addition of Ni and for a current density of 9.2 mAcm⁻², 100% HMF conversion and 93.3% FDCA yield were obtained at a potential of 1.45 V vs. RHE (entry 16, Table 2).¹⁸⁵ CuMn₂O₄ ¹⁸⁶ spinel introduces a large number of oxygen and metal vacancies in situ on the surface; however, when used directly as an electrocatalyst, it has a poor catalytic performance due to low exposure of active sites. Zhu et al.¹⁶² subjected CuMn₂O₄ to selective ammonia corrosion treatment to remove some Cu atoms and expose Mn sites. At a current density of 20 mA cm⁻², 100% conversion and 96% FDCA yield were achieved (entry 17, Table 2). Deng et al.¹⁸⁷ designed a Cu_xS@NiCo–LDH core–shell nanoarray bifunctional electrocatalyst, using highly conductive Cu_xS as the core to facilitate charge transfer from the substrate, Cu, to the NiCo–LDH catalytic active layer and to expose a large number of active sites (entry 19, Table 2), resulting in a cooperative three-metal electrode that gave high FDCA yields (99%) and FE (95%) values. Dual-electrode coupling of the electrolytic system can generate H₂ and FDCA at the same time with only 1.34 V vs. RHE.¹⁸⁷

Mo-based materials can promote the oxidation of HMF by forming a unique heterojunction interface with other components, thus inducing electron transfer, which provides a new idea for electrooxidation of furan compounds.^188,189 A heterogeneous recyclable bimetallic CuMoO₄ catalyst was applied to furfural for selective oxidation to 2(5H)-furanone and maleic acid (MAc). The synergistic effect of CuMo promoted redox of Mo⁶⁺–Mo⁵⁺–Mo⁴⁺ through redox of Cu⁺/Cu²⁺, with a furfural conversion of 99%, giving a 2(5H)-furanone yield of up to 66% or a maleic acid yield of over 74%.¹⁹⁰ However, Cu-based catalysts are readily deactivated during electrolysis and have limited current densities of about 20 mAcm⁻² at applied potentials or at high overpotentials, which restricts their industrial application.

Vanadium-based materials are thought to be suitable electrocatalysts due to the multiple oxidation states of V.¹⁹¹ Li et al.¹⁹² demonstrated that 3D vanadium nitride hollow nanospheres (3D VN HNs) were good electrocatalytic materials for promoting HMF oxidation and obtained 98% HMF conversion, 96% FDCA selectivity, and 84% FE. Liang et al.¹⁹³ synthesized the bifunctional electrocatalyst Ni₃N–V₂O₃ by a hot ammonia method. By redistributing charge to promote the interface effect between Ni and vanadium oxide, hydrogen adsorption on Ni metal was weakened, and oxidation of biomass-related compounds with a high FE (100%) coupled with film-free hydrogen production was realized. Gao et al.¹⁹⁴ prepared the NiVCo–LDH electrocatalyst by co-doping vanadium and cobalt and carried out HMF oxidation at a potential of 1.37 V vs. RHE through diatomic co-modulation, and obtained FDCA yields of up to 99.7% for 10 consecutive cycles (entry 20, Table 2). In summary, achieving high current densities at low potentials with the metals and methods described remains challenging. To overcome these issues for industrial application, more types of electrocatalysts, such as mixed metal/oxide heterojunctions, new media, and non-metallic materials, need to be developed. Typical electrooxidation properties of non-noble metal electrodes are shown in Table 2.

2.3. Non-metallic electrodes

To overcome some of the inherent disadvantages of metal-based electrodes (e.g. cost, durability, toxicity),¹⁹⁶ electrodes fabricated from non-metallic materials (e.g. carbon, organic compounds) have been proposed for the electrocatalytic oxidation of biomass-derived compounds.¹⁹⁷ Carbon-based materials have high electronic conductivity, good stability, low cost, and their configuration can be readily modified into carbon nanotubes,^198,199 nanosheets,²⁰⁰ graphene,²⁰¹ or porous carbon,^75,202,203 all of which have future applications in electrocatalysis (Fig. 6).

Fig. 6 Types and structures of carbon-based metal-free electrocatalytic electrodes. C-MFECs: carbon-based metal-free electrocatalysts.

2.3.1. Carbon-based electrocatalysts. Research on carbon-based electrocatalysts has focused on doping with heteroatoms and defect engineering for enhanced performance.²⁰⁴ Defects in carbon nanomaterials, such as pore defects, can disrupt electron–hole symmetry and introduce active sites, resulting in high electrocatalytic activity.^166,205,206 Qin et al.²⁰² used B/N co-doped porous carbon materials with abundant point defects and polar pore wall structures to achieve 71% HMF conversion and 57% FDCA yield. The rich point defects in grain boundaries provided a large number of unpaired electrons and frustrated Lewis pairs (FLPs), leading to improved catalytic activity.²⁰² Reports on carbon materials for electrochemical oxidation of HMF, however, are still quite limited, so the development of carbon-based electrocatalysts has yet to be fully realized. Besides doping and introducing defects, the construction of 3D carbon nanostructures through hybrid designs is a means to create highly active non-metallic carbon catalysts.¹⁹⁸

Although the use of carbon-based materials can improve the electrocatalytic activity of an electrode (increased conductivity, active sites, durability), the performance of carbon-based electrocatalysts still tends to be lower than that of metal-based electrocatalysts. However, the combination of carbon nanostructures with transition metals to form hybrid materials can improve the electrocatalytic activity of carbon-based electrodes. Under severe conditions of a strong alkaline electrolyte or high overpotential, bare transition metal electrocatalysts are typically unstable. Encapsulating transition metal species in carbon shells inhibits corrosion while promoting electron transfer between carbon and the metal.²⁰⁷ High-activity air cathodes made of Ni₃Fe nanoparticles embedded in porous N-doped carbon sheets (Ni₃Fe/N–C) have been reported,²⁰⁸ which enable zinc–air batteries to cycle continuously for up to 420 h at a discharge–charge current density of 10 mA cm⁻². Gupta et al.²⁰⁹ showed that in situ formed FeCoNi-decorated N-doped graphene tubes (FeCoNi/N–GT) had thicker tube walls and higher crystallinity than either Fe/N–GT or NiFe/GT, which contributed to improved electrode stability.

2.3.2. TEMPO and its derivatives. TEMPO, which has the IUPAC name of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, and its derivatives are a class of piperidine nitrogen oxygen compounds that can trap free radicals and quench singlet oxygen. This makes TEMPO and TEMPO-related compounds very effective oxidation catalysts and consequently, they represent one of the main non-metallic materials for the electrocatalytic oxidation of HMF.^210–213 The performance of TEMPO and 4-acetamido-TEMPO (ACT) in electrocatalytic oxidation has been reported.^214,215 Carbon felt was used as the working electrode, TEMPO, which is a compound soluble in the electrolyte, was used as an activator, and 0.1 M NaOH (pH 13) and 0.5 M boric acid (pH 9.2) were used as the cathode and anode buffer, respectively. At an overpotential of 1.54 V vs. RHE, 100% HMF conversion and 99% FDCA yield were obtained (entry 2, Table 3).²¹⁵ Typically, when TEMPO is used for a homogeneous reaction, the presence of a chemical oxidizer rapidly regenerates TEMPO, so only a catalytic amount of TEMPO is required. However, in the electrocatalytic oxidation of HMF, the regeneration of TEMPO can only be achieved through oxidation reactions on the electrode surface, so a considerable amount of TEMPO is required to ensure a steady oxidation rate in concentrated HMF solutions.²¹⁶ To address this issue, Cardiel et al.²¹⁶ studied the stability and electrochemical and electrocatalytic properties of TEMPO and ACT and determined that HMF could be efficiently oxidized under mild alkaline conditions (pH 9–10), while other electrocatalysts typically required the use of an electrolyte in the pH range of 13–15 and lost their catalytic activity at lower pH.^123,217 The reason for this is that the homogeneous process of electrocatalytic oxidation using TEMPO as a mediator results in improved catalytic activity, because the catalyst and substrate facilitate electron transfer on the electrode surface and in the same electrolyte.²¹⁸ In addition, it can be more economical to use ACT as a mediator, considering that it is more soluble and less expensive per kilogram than TEMPO.^211,215 Chadderdon et al.²¹⁹ performed similar experiments and used carbon felt as the working electrode and ACT as the mediator and obtained 97% FDCA yield and 93.5% FE by electrolysis with 0.7–0.9 V vs. RHE potential in the same boric acid buffer (entry 3, Table 3) (Fig. 7). Although the electrocatalytic oxidation of TEMPO and its derived catalysts gave high FDCA selectivities and yields due to the high water solubility of ACT, a homogeneous reaction system is formed, which results in issues related to catalyst recovery and product separation.²²⁰ TEMPO-mediated electrocatalytic oxidation methods have been reported for the selective conversion of HMF into DFF; TEMPO catalysts can be effectively recovered via nanofiltration.²²¹ Kisszekelyi et al.²²¹ used organic solvents such as acetonitrile and γ-valerolactone instead of an aqueous solution, with TEMPO as a catalyst, and synthesized a new homogeneous size amplified C₃-symmetric tris-TEMPO derivative in which the synergistic effect of TEMPO and lutidine could be observed.²²¹ While ensuring high yield and selectivity, HMF was converted into DFF, and the catalyst could be recovered by an organic solvent nanofiltration (OSN) method, with a separation yield of 78% and a selectivity of 100%.

Fig. 7 (a) Schematic of 4-acetamido-TEMPO (ACT)-mediated electrochemical oxidation of 5-hydroxymethylfurfural; (b) synthesis of the size-enlarged Hub¹-TEMPO catalyst in tetrahydrofuran (THF) at room temperature (RT). Reproduced from ref. 222 with permission from American Chemical Society, copyright 2018.

Table 3 Electrocatalytic performance of non-metallic catalysts for producing FDCA by electrocatalytic oxidation of HMF

Entry	Electrocatalyst^a	Electrolyte^b	Initial HMF (mM)	Current density (mA cm^−2)	Potential (V vs. RHE)	Conv. (%)	Yield (%)	FE (%)	Ref.
a BNC: B and N co-doped porous carbon; TEMPO: 2,2,6,6-tetramethylpiperidin-1-yloxyl; ACT: 4-acetamidoTEMPO; C®3TEMPO: a new homogeneous size-enlarged C3-symmetric tris-TEMPO derivative. b PolarClean (Rhodiasolv): methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate.
1	BNC	0.1 M NaOH	5	—	1.9	71	57	15.2	202
2	TEMPO	0.5 M boric acid	5	0.48	1.54	100	99	99.9	215
3	ACT	0.5 M boric acid	20	—	0.9	98	97	93.5	219
4	C®3TEMPO	PolarClean	—	—	1.28	100	98	—	221

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2.4. Catalyst design strategies

Regardless of the catalyst type, optimizing key structural and compositional features, such as crystal configuration,²²³ particle morphology,²²⁴ electronic structure,¹⁷⁰ and defect chemistry,²²⁵ have emerged as important properties for addressing issues like sluggish reaction kinetics, limited active site accessibility, and mass transfer bottlenecks. The crystal configuration of catalysts profoundly influences their active sites and energy states (Fig. 8). Engineering the crystal surface can optimize electron transport, increase the density of surface-active sites, and thereby enhance electrocatalytic activity, as previously demonstrated.¹⁵⁸ Furthermore, altering the temperature or environmental conditions may induce phase transitions or structural rearrangements in catalysts, dynamically modifying their crystal structures and surface properties. Exploiting such structural regulation enables precise control over catalytic activity, enhances material responsiveness to electrochemical stimuli and boosts electrocatalytic performance.

Fig. 8 Strategies to enhance electrocatalyst performance.

Catalyst particle size and morphology are intrinsically linked to specific surface area.²²⁶ Catalysts with smaller particle sizes generally exhibit higher specific surface areas and more accessible surface–active reaction sites. The design of morphologies such as nanorods, nanoplates, or porous architectures can further amplify the specific surface area and create efficient transport pathways for reactants.²²⁷ Strategic engineering of pore structures, for instance, effectively increases the contact interface between catalysts and the electrolyte, enhancing mass transfer kinetics. For instance, mesoporous materials with multistage pore architectures can expedite the diffusion of reactants and products alike, leading to enhanced mass transfer rates and ultimately increasing electrocatalytic performance.

Doping with heteroatoms represents a robust strategy to enhance electrocatalysis by modulating electron valence states and reconfiguring the electronic structure of active sites.²²⁸ For example, Xu et al.²²⁹ showed that Cu doping introduced abundant Lewis acid sites, altering the electronic configuration of active sites and fine-tuning the local coordination environment around Ni centers to facilitate HMF adsorption and accelerate reaction kinetics. Non-metallic doping (N, P, S) can also be used to enhance catalytic performance.¹¹⁶ Precise control of impurity contents and doping locations not only increases the number of catalytic active sites, but also improves electron transfer efficiency, making the catalytic process more effective at electrooxidation.

Defect and vacancy engineering plays a critical role in tailoring catalyst electronic structures and surface chemistry, directly impacting performance. In oxide-based catalysts, oxygen vacancies or structural defects in metal oxides serve dual functions: they provide additional active sites and improve electrical conductivity, both of which synergistically enhance catalytic efficiency.²³⁰

3. Electrochemical reactor

The configuration of an electrolytic cell enables laboratory-determined conditions to be studied on a continuous or large scale. Four basic types of electrochemical reactors are analyzed in this section, according to their application to the electrooxidation of furans.

3.1. Classification of reactors

Electrochemical reactor types suitable for the electrooxidation of furans (Fig. 9) include (i) a single electrolysis cell,^231–233 (ii) an H-type electrolytic cell,^234–236 (iii) a flow cell^237–239 or (iv) an MEA-based electrolytic cell.^240–242 The single electrolysis cell is the simplest type of reactor that does not require an ion exchange membrane (Fig. 9a). However, liquid products generated may react with each other, which reduces overall reaction efficiency and increases separation costs.^243–245 The H-type electrolytic cell reactor divides the cathode and anode contents with an ion exchange membrane or salt bridge, which enables precise regulation of the positive and negative electrode potentials (Fig. 9b). Moreover, ion exchange membranes prevent the mutual influence of reaction products;^137,141,246 however, high energy consumption and relatively low efficiency restrict their use to laboratory settings.²⁴⁷ Compared with H-type cells, the flow cell uses a separator instead of an ion exchange membrane and its design is simple, modular and has a good service life and operational characteristics (Fig. 9c). The lack of an ion exchange membrane in the flow cell enhances mass transfer, reduces ohmic loss, and enables precise voltage control.^117,248 However, issues in the flow cell reactor for electrooxidation include a large ohmic loss and ion imbalance.²⁴⁹ Hauke et al.²⁵⁰ developed a zero-gap MEA-type electrolyzer, which consisted of a single pass flow regime that could recycle the solution through the cell (Fig. 9d). By recycling the reaction solution, efficient use of reactants could be achieved along with stable operating conditions, which enhanced HMF conversion (100%), FDCA selectivity (100%), and FE (100%) at a sufficient current density (>150 mA cm⁻²). Special zero-gap components reduced interface contact resistance, promoted ion transport and the start-up response and provided a low stable voltage for extended periods, all of which were favorable for industrial applications.^251,252

Fig. 9 Schematic illustration of (a) single electrolytic cell, (b) H-type electrolytic cell, (c) flow cell and (d) MEA-based electrolytic cell. (e) Schematic representation of the electrochemical undivided-cell microreactor; (f) process scheme of integrated two-step synthesis (conditions: 0.5 mol L⁻¹ of fructose, 0.2 mol L⁻¹ of H₂SO₄, 500 mL min⁻¹ flow rate, current density of 15 mA cm⁻²).

In terms of reactor design, the MEA electrolytic cell seems to have favorable characteristics for industrial production. The MEA electrolytic cell typically consists of a bipolar plate (BPP), fluid collector, porous transport layer (PTL), catalyst layer (CL), and ion exchange membrane. The catalyst coated membrane (CCM) electrode and porous transport layer electrode (PTE) are the primary components of the electrolytic cell.

For the CCM, the catalyst can be attached to the membrane using spraying or decal transfer methods,²⁵³ which helps to reduce interface contact resistance and enhance ion transport. The spraying technique is cost-effective and straightforward, but can cause membrane swelling.²⁵⁴ The decal transfer method provides better interface contact with the membrane, but existing procedures are too complex and require simplification.²⁵⁵ For the PTE, coating and sputtering are the most common methods for construction.²⁵⁵ Coating methods tend to produce a uniform and flat catalyst distribution. Sputtering enables precise control over the thickness of 20–40 μm and loading of the catalyst layer on the membrane and has broad application prospects.²⁵⁵

The PTL is crucial in HMF conversion processes that do not involve gas production due to its effective mass transfer channels. The PTL material is sensitive to the acidity and alkalinity of the electrolyte.²⁵⁶ In alkaline or neutral electrolytes (1 M KOH/1 M PBS), metal foam substrates such as nickel foam and copper foam perform well.²⁵⁰ However, in acidic electrolytes (0.5 M H₂SO₄), especially under high current densities (>500 mA cm⁻²), the metal substrate will gradually corrode, leading to structural collapse and failure of mass transfer. Currently, Ti fiber is used as the anode PTL under acidic conditions (0.5 M H₂SO₄).²⁵⁷ However, under long-term operation, the substrate surface will form a passivation layer that blocks electron transfer, resulting in an increase in ohmic impedance and a decrease in efficiency.²⁵⁸

Therefore, research on anti-corrosion coatings should be further deepened, including precious metals (Pt, Au, Ir) and non-precious metals (NbN).²⁵⁹ It is worth noting that the PTL material of the cathode only requires carbon paper, which is readily available. The bipolar plate accounts for a major part of the cost of the MEA electrolytic cell, and particularly under acidic conditions, it is necessary to use Ti metal, which is expensive. Similarly, BPP requires improvement in corrosion resistance to enable it to be used economically under acidic conditions. Stiber et al.²⁶⁰ coated stainless steel BPP and PTL with Ti and Nb/Ti non-precious metal coatings, respectively. Long-term stability at high temperatures and large currents has been confirmed, making it possible to significantly reduce the cost of Ti–BPPs and Ti–PTL.

Liu et al.²⁶¹ developed a high-performance flow cell using oxide-derived silver cathodes. The cell voltage could be reduced by 5.5 V vs. RHE (∼7.5–2.0 V) compared with H-type electrolysis cells, which was equivalent to an energy efficiency increase of more than four-fold when operating at a current density of 10 mA cm⁻². Chen et al.²⁶² achieved 79.2% yield of DFF in a continuous flow reactor using Cu-based electrocatalysts. Li et al.²⁶³ achieved the electrochemical oxidation of benzyl alcohol coupled with cathodic hydrogen production in a flow cell using an extremely low initial potential of 0.7 V vs. RHE, but the current density was limited (160 mA cm⁻²). Delparish et al.²⁶⁴ developed a continuous-flow microreactor for electro-oxidation of HMF using parallel plate electrodes made of Ni and an 8-channel spacer (Fig. 9e). The electrochemical reactor could be used over a potential range of 1.6–1.8 V vs. RHE and allowed flow rates of 0.1–4 mL min⁻¹. Harhues et al.²⁶⁵ designed a biphasic electrochemical reactor that paired electrochemical oxidation with fructose dehydration, reducing the need for intermediate product separation (Fig. 9). In the system, fructose dehydration to HMF proceeded with in situ extraction into an organic solvent phase (2-MTHF), while HMF was continuously extracted into the aqueous phase and oxidized with a Ni(OH)₂/NiOOH-coated nickel-foam electrode (Fig. 9f) to achieve close to 80% yield of FDCA.

4. Pairing electrocatalysis with HMF oxidation

Electrocatalysis involves the pairing of two half-reactions: anodic oxidation and cathodic reduction.^266,267 The pairing of the valorization of HMF with other half-reactions provides a practical way to improve the energy conversion efficiency and obtain high-value products.²⁶⁸ However, the voltage required for the paired reaction of the anode and cathode is not the same, and cannot be controlled independently in the two-electrode system. Therefore, it is necessary to research coupling reactions that have similar kinetics and charge transfer numbers.

4.1. Pairing with HER

Hydrogen, as a secondary energy carrier, has the advantages of light specific gravity, high energy density and only water vapor as the byproduct when used in fuel cells,²⁶⁹ and it is considered as a substitute for carbon-based fuels in future energy systems.^268,270 Electrocatalytic decomposition of water to produce H₂ is referred to as green hydrogen when renewable electricity is used and is attracting attention.^271,272 Water decomposition involves two tightly paired half-reactions: oxygen evolution (OER) and hydrogen evolution (HER). The kinetics of the OER are slow and require a high overpotential, resulting in a low energy conversion efficiency.^161,273 This has spurred investigations into alternative reactions, such as the oxidation of biomass-derived compounds (e.g. HMF) to improve the energy conversion efficiency. Coupling electrocatalytic HMF oxidation with the HER in the field of new energy could remove the bottleneck in electrocatalytic water splitting systems and effectively lower energy consumption. Xie et al.²³⁶ prepared a self-supporting Co(OH)₂–CeO₂ catalyst and used it as an anode for the electrooxidation of HMF and the HER in a two-cell electrolytic system. Under neutral conditions (pH 7), Co(OH)₂–CeO₂ achieved the selective dual electron electrocatalytic conversion of HMF to HMFCA at 1.4 V vs. RHE (selectivity 89.4%), and the coupled hydrogen production of the cathode at 1.4 V vs. RHE reached 114.4 μmol cm⁻² h⁻¹, which was 4.1 times that of the water decomposition system. This indicates that HMF oxidation can replace water oxidation in green hydrogen production schemes. Thapa et al.²⁷⁴ prepared ternary heterostructured electrocatalysts to pair the anodic oxidation of HMF with the cathodic HER. When HMF was present, a current density of 50 mA cm⁻² was possible at a potential of 1.8 V vs. RHE and overall water decomposition could be achieved. Moreover, when the HMF concentration was increased to 10 mM, the same current density was possible at 1.62 V vs. RHE, which was 180 mV lower than the total water decomposition conditions, indicating that the coupled anode reaction could improve the energy efficiency of the entire battery.²⁷⁴

4.2. Pairing with CRR

Electrocatalytic CO₂ reduction is a possible way to improve carbon recycling and reduce carbon emissions.^275,276 Coupling HMF value-added reactions with CO₂ electroreduction offers an energy efficient enhancement method in chemical production.^53,277 Bi et al.²⁷⁸ developed an electrolytic cell that used PdO_x/ZIF-8 as the cathode and PdO as the anode, which only required 1.06 V vs. RHE initial voltage to convert CO₂ into CO and HMF to FDCA. At a current density of 103.5 mA cm⁻², 97.0% FE and 84.3% organic acid yield were achieved. This work opens up new ways to efficiently convert CO₂ into CO and to reduce CO₂ emissions and oxidize biomass into chemicals simultaneously. Hauke et al.²⁷⁹ designed a paired low-temperature bipolar membrane (BPM)-based HMF oxidation and CO₂ electrolytic cell to enable application at industry-relevant current densities (200 mA cm⁻²). Ye et al.²⁸⁰ used oxygen-vacancy-rich indium hydroxide (InOOH–OV) as an electrocatalyst for integrating the CRR and HOR into a single electrolytic cell with asymmetric pH values. The issue of pH mismatch between the anode electrolyte (1 M KOH) and cathode electrolyte (0.1 M KHCO₃) of the HOR was addressed by use of a bipolar membrane. At a voltage of 1.48 V vs. RHE, the cathode CO₂ reduction reaction achieved a formate yield of 90% and 92.6% FE, while the anode HMF oxidation reaction obtained an FDCA yield of 87.5% and 90% FE.

4.3. Pairing with organic electrocatalytic hydrogenation (ECH)

Combining organic oxidation and rate-matched organic hydrogenation using bifunctional electrodes in a shared electrolyte is another way to improve catalytic efficiency and electrical economy.^281–283 Organic electrocatalytic hydrogenation (ECH) with water as the donor is considered to be green organic reduction technology with potential applications in the production of value-added chemicals.²⁸⁴ Yang et al.²⁸⁵ prepared a FeP–MoP/FF high efficiency cathode catalyst grown in situ on foam-based iron for the hydrogenation of 4-nitrobenzoic acid. FeP–MoP/FF exhibited low potentials (1.13 and 1.59 V vs. RHE) for current densities of 10 mA cm⁻² and 100 mA cm⁻² compared with the potential required for driving the HER. Ren et al.²⁸⁶ prepared NiFe layered double hydroxides (NiFe–LDH) rich in oxygen vacancies and showed 97% FE of p-aminophenol at high p-nitrophenol (p-NP) concentrations (100 mM). Yuan et al.²⁸⁷ examined the feasibility of electrochemical Clemmensen reduction, elucidating factors that influenced electrochemical hydrolysis and hydrogenation pathways of Zn. Li et al.¹⁹² assembled a membrane electrode (MEA) by spraying two different types of electrocatalysts onto a bipolar membrane. Using VN and Pd/VN coated onto a bipolar film as the anode and cathode, respectively, a constant current of 100 mA and a voltage range of 2.5–3.0 V vs. RHE resulted in HMF conversions of 92% at the cathode (production of FDCA) and 87% at the anode (production of DHMTHF). Zhang et al.²⁸⁸ constructed a paired system with NiB_x as both the cathode and anode working electrodes, which combined with the oxidation of HMF and the hydrogenation of p-nitrophenol (p-NP) in 1.0 M KOH solution. Since both cathode and anode reactions involve six-electron transfer, the effect of current density mismatch between cathode and anode was lowered.

4.4. Pairing with HMF reduction

Reduction of HMF mainly produces biofuels (N,N-dimethylfuran), polymer precursors (2,5-furandimethanol), and organic solvents.^289,290 Roylance et al.²⁹¹ modified a silver electrode through a galvanic displacement method for electrocatalytic HMF reduction. The yield and FE of 2,5-bis (hydroxymethyl) furan (BHMF) in a slightly alkaline solution were close to 99% at −1.3 V vs. Ag/AgCl. Zhang et al.²⁹² proposed an electrocatalytic reduction amination and simultaneous oxidation (ERAO) route to convert 5-hydroxymethylfurfural (HMF) into 2-hydroxymethyl-5-(methylethanolamine) furan (HEMF) and 2,5-furan dimethyl acid (FDCA) on the cathode and anode sides, respectively. The N–Ti–O active center and porous structure exposed by Ti–MOF provided additional channels for internal sites, and 68% HMF conversion was obtained after 6 h of reaction with 30% FDCA yield. de Luna et al.²⁹³ used CeO₂-coated copper foam for electrocatalytic hydrogenation of HMF to BHMF by electrodeposition, which speeded up the reaction kinetics and achieved nearly 100% selectivity of FDCA and BHMF at −0.51 V vs. RHE. Therefore, pairing HMF reduction with HMF oxidation can yield two value-added products and avoid the slow kinetics of the OER. However, there is a mismatch between the optimal potential and current density of the two half-cell reactions in paired electrocatalysis, and well-developed redox mediators are the key to resolve mismatches. It has been reported that TEMPO and its derivatives like ACT can be used as redox mediators for HMF electrooxidation in alkaline electrolytes due to their rapid redox kinetics, high solubility in water, remarkable stability, and suitable redox potentials.²⁹⁴ Chadderdon et al.²¹⁹ studied the electrohydrogenation of HMF to 2,5-bis (hydroxymethyl) furan (BHMF) with the Ag/C cathode catalyst at mild pH. ACT was employed as the redox mediator for the electrooxidation of HMF to FDCA. Electrocatalytic HMF conversion in the paired cells achieved high yields of BHMF and FDCA (85% and 98%, respectively) and a combined electron efficiency of 187%, which was nearly twice the efficiency compared to the unpaired cells.

5. Electrolyte-related factors

5.1. Electrolyte concentration

Electrolyte concentration and pH greatly affect the kinetics of electrooxidation of HMF to FDCA and electrochemical parameters.^154,253 The oxidation reaction of HMF involves the interaction of nucleophilic groups (hydroxyl and aldehyde groups) with active hydrogen at catalytically active sites.¹²³ There are two chemical routes for the oxidation of HMF molecules and each depends on the sequence of the reaction for its two functional groups. When the aldehyde group preferentially reacts, the aldehyde is oxidized to an acid group to generate HMFCA, and when the hydroxymethyl group is preferentially oxidized, HMF is oxidized to DFF. DFF and HMFCA are further oxidized to FFCA, which then forms FDCA (Fig. 9d).²⁹⁵ However, the infrared spectra of HMF and DFF are highly similar, which complicates the determination of the HMF oxidation pathway. Product detection with in situ detection techniques can help to track and analyze reaction intermediates and therefore are necessary for elucidating reaction pathways.¹²⁷ Zhang et al.¹²⁷ investigated changes in interfacial molecules during the electrocatalytic oxidation of HMF with a sum frequency generation (SFG) technique. As illustrated in Fig. 10a–c, only the signals corresponding to the intermediate compound HMFCA were detected during electrolysis in a strongly alkaline environment. This result is direct confirmation that the electrocatalytic oxidation of HMF proceeds via formation of the intermediate HMFCA (path I). In a highly concentrated alkaline electrolyte (1 M KOH), HMF undergoes electrooxidation via path I with high selectivity (95%) and FE (90%). In fact, in a low concentration alkaline electrolyte (1 M KOH, pH < 13), HMF oxidation occurs via formation of the intermediate DFF (path II) (Fig. 10d).⁶⁶ Fu et al.²⁹⁶ used in situ Raman and FTIR to capture gem-diol intermediates during HMF oxidation, and directly observed key intermediates of HMF oxidation, revealing the long-debated reaction mechanism. In an alkaline medium, HMF forms two reversible gem-diol intermediates (DHFm⁻ and DHFm²⁻). DHFm²⁻ produces H₂ through hydrogen transfer (H⁻) while DHFm⁻ produces H₂O through proton transfer (H). Wei et al.²⁹⁷ used in situ FTIR to monitor HMF adsorption and the intermediate conversion process on the Au/CeO₂ catalyst surface. The oxidation path of HMF was revealed and the role of oxygen vacancies in accelerating C–H bond breakage was clarified, which promoted the refinement of reaction path analysis. Fan et al.²⁹⁸ used in situ Raman spectroscopy to trace the valence change of the CuO–SO₄²⁻ catalyst during the reaction. A reversible transformation from Cu^II–O to Cu^III–OOH was observed, confirming the hypervalent copper species as the active site. In addition, the adsorption and conversion of surface intermediates were monitored by in situ infrared spectroscopy, which revealed that surface sulfate (SO₄²⁻) promoted the dehydrogenation of HMF to HMF–H* intermediates by weakening the Cu–OH bond.

Fig. 10 (a) HPLC chromatograms taken at various electrolysis charges; (b) sum frequency generation (SFG) results after running the cell at several potentials for 90 min and (c) at different times at 1.45 V vs. RHE. Reproduced from ref. 127 with permission from Wiley, copyright 2019. (d) Reaction pathways of HMF oxidation on electrocatalysts in alkaline media; reaction mechanism of direct oxidation at (e) pH < 13 and (f) pH ≥ 13. Reproduced from ref. 301 with permission from Wiley, copyright 2023.

In the process of HMF oxidation, the external potential directly drives the oxidation reaction, and protons in the substrate molecules are captured by OH^– adsorbed onto the electrode surface, and a electron transfer effect occurs at the same time.²⁹⁹ Under weakly alkaline conditions, the hydroxymethyl group of HMF is adsorbed onto the electrode surface, and OH^– in the electrolyte activates and guides deprotonation of the C–H and O–H bonds of HMF, thus producing DFF intermediates. A geminal diol is subsequently created by nucleophilic addition between the aldehyde group and H₂O. The C–H and O–H bonds of the geminal diol are then deprotonated to create FFCA after being activated by OH^–. FDCA is produced by repeating the two processes of nucleophilic addition and aldehyde deprotonation (Fig. 10e). In an electrolyte having a pH ≥ 13, the aldehyde group of HMF will selectively adsorb onto the catalyst surface and bound water to produce bisphenol, and then electrocatalytic dehydrogenation will occur under the activation of OH^– in the electrolyte to form HMFCA intermediates. Subsequently, hydroxymethyl deprotonation of HMF forms FFCA, and finally, the nucleophilic addition and dehydrogenation steps are repeated to form FDCA (Fig. 10f).²⁹⁹ Ge et al.³⁰⁰ reported an electrocatalyst (Ru₁–NiO) for the selective electrooxidation of alcohols to aldehydes in a neutral electrolyte. In the HMF oxidation reaction, Ru was found to promote the oxidation of HMF in neutral media via hydrolysis to produce OH*, and a DFF selectivity of 90% was reported.³⁰⁰ Notably, Ru₁–NiO could be extended to selectively electrooxidize a range of alcohols to the corresponding aldehydes; this is generally difficult to achieve in alkaline media. The study of the dissociation energy of HMFC–H/O–H bonds can be used to evaluate the catalytic performance of HMF.

The dissociation energy of HMFC–H/O–H bonds can be studied to evaluate the performance of electrocatalytic systems. In addition to potential, the adsorption of hydroxyl groups is an important factor in catalyzing the activation of C–H/O–H bonds.^83,202 Zhang et al.¹²⁷ found that increasing the KOH concentration from 0.1 M to 2 M caused the current density to increase for a given potential, resulting in an improvement in the HMF electrooxidation activity. Increasing the electrolyte concentration from 0.1 M to 2 M KOH shifted the linear sweep voltammetry (LSV) curves negatively and improved the activity,³⁰² possibly due to increased conductivity and improved catalyst–substrate contact or oxidation conditions.³⁰³ Competitive adsorption of high concentrations of OH^– and HMF can inhibit conversion of the intermediates HMFCA or DFF into FDCA, thus reducing the catalytic activity.³⁰⁴ Woellner et al.³⁰⁵ investigated non-electrochemical losses (all compounds not in the reaction network) of HMF in KOH electrolytes at KOH concentrations of 0.01, 0.1, 1, and 3 M and found that the degradation of HMF at low KOH concentrations (≤1 M) was less than 10%, while HMF was readily converted into humins (degradation greater than 30%) at high KOH concentrations (≥1 M).¹⁹² An optimal concentration of KOH electrolyte for producing FDCA by electrocatalytic oxidation of HMF was proposed, 1.0 M KOH, which could achieve low HMF degradation (<10%) and a high FDCA yield (>90%).³⁰⁵ Electrocatalytic systems have been developed for the selective oxidation of HMF in acidic electrolytes with electrocatalysts such as mesoporous δ-MnO₂,³⁰⁶ MnO_x,¹⁵⁹ MOF-derived Ag/AgO_x–CN_x,³⁰⁷ or RuO₂/MnO₂/CNT.³⁰⁸ However, there are still challenges for the application of electrocatalytic systems under acidic conditions that can be summarized as being low product selectivity and the formation of byproducts (maleic acid, acid-soluble humins).³⁰⁶

5.2. HMF concentration

Substrate concentration is an important factor in HMF electrochemical oxidation that does not necessarily coincide with general reaction kinetics most likely due to the decomposition of HMF into humins, which adversely affects the catalytic activity.³⁰⁹ Zhang et al.¹⁶³ found that with an increase in HMF concentration, the catalytic activity was increased and higher current densities were possible for a given potential.¹⁶³ Pang et al.³¹⁰ demonstrated that there was a linear relationship between the initial HMF concentration and current density, which indicated that HMFOR belonged to the category of diffusion-limited reactions. Furthermore, an increase in HMF concentration leads to a decrease in the charge transfer impedance between the catalyst and HMF, thus increasing the reaction activity. When the concentration of HMF is increased from 10 mM to 50 mM in 1 M KOH, charge transfer impedance between the catalyst and HMF decreases and the reaction activity increases.³¹¹ However, further increasing the HMF concentration to 100 mM causes excessive adsorption of HMF molecules, which cover the active sites and hinder adsorption of OH^–, leading to decreases in HMF conversion and FDCA selectivity.³¹¹ Liu et al.¹⁷⁸ used a first-order kinetic model to perform electrooxidation of HMF for a series of substrate concentrations and then deduced that there was an optimal concentration of reactants for the electrooxidation of HMF at a given potential. Increasing the initial concentration of HMF leads to humin formation, which decreases the purity of FDCA.³¹² Challenges for the industrial production of FDCA will include the requirement for the processing of feed streams and product streams at high concentrations to ensure cost-effectiveness and methods to reduce the number of energy-intensive product separation and recovery steps.³¹³

Several studies have used HMF at high concentrations for the electrooxidation production of FDCA.^256,314 Zhou et al.³¹⁵ developed a single-pass continuous flow reactor system and studied the electrooxidation of HMF at high concentrations (200 mM) to achieve high FDCA selectivity (91.3%). Single-pass continuous flow reactors (SPCFRs) have the characteristics of a high electrode area/electrolyte product ratio, short duration of the reactor path, and separation of the substrate and alkaline solution feed, thus inhibiting non-Faraday degradation to a large extent; this allowed FDCA concentrations of 556.9 mM to be obtained with 96.9% selectively from an HMF substrate concentration of 600 mM.³¹⁵ Therefore, the design of continuous flow electrochemical reactors can effectively overcome the shortcomings of high concentration substrate degradation and effectively promote mass transfer and diffusion; this is conducive to continuous and large-scale industrial production.

6. Separation methods

The physicochemical properties of HMF and FDCA (Table 4) show that HMF is freely soluble in water, while FDCA has a water solubility of 1 g L⁻¹ at 291.15 K and FDCA has high solubility in highly alkaline solutions (pH ≥ 13), while it is insoluble in highly acidic solutions (pH < 2, ca. w_FDCA of 10⁻³) (Fig. 11a).^307,311,316 Because HMF is highly soluble in both alkaline and acidic solutions, its presence in reaction solutions does not directly affect the separation of FDCA under acidic conditions;³¹⁶ however, FDCA solubility can be affected by HMF concentration. FDCA will crystallize from water by lowering the solution pH that causes FDCA to precipitate, which enables it to be filtered from solution.³⁰⁷ However, simple precipitation of FDCA from solution requires additional acid and produces salts, which increases the cost of FDCA separation.³¹⁷ An acid reaction solution process was developed that enabled FDCA to be readily isolated from aqueous solutions (Fig. 11b).¹⁵⁹ The pK_a of HMF is 12.8, which means that HMF acts as a base, while FDCA is diprotic (pK_a values of 2.6 and 3.55), which means that FDCA is completely protonated at pH < 2 (Table 4).³¹⁸

Fig. 11 (a) Solid solubility of FDCA in water versus pH at 308.2 K and 1 bar. Reproduced from ref. 316 with permission from American Chemical Society, copyright 2018. (b) Electrooxidation of HMF to FDCA with in situ isolation of FDCA. (c) Data from the literature on 2,5-furandicarboxylic acid solubilities in pure water, acetic acid, methanol, 1-butanol, isobutanol, MIBK, ethyl acetate, acetonitrile, 1,4-dioxane (DX), tetrahydrofuran (THF), 1,2-dimethoxyethane (DME), and diethylene glycol dimethyl ether (DGDE) at different temperatures. Reproduced from ref. 320 with permission from American Chemical Society, copyright 2023; reproduced from ref. 321 with permission from American Chemical Society, copyright 2018; reproduced from ref. 322 with permission from American Chemical Society, copyright 2018. (d) Mole fraction solubility (x) of FDCA in acetonitrile–water or acetic acid–water mixtures plotted versus water mole fraction (x_water) on a solute-free basis at 313.15 K. Reproduced from ref. 321 with permission from American Chemical Society, copyright 2018.

Table 4 Physicochemical properties (melting point temperature, T_m; normal boiling point, T_b; octanol–water partition coefficient, log P) of 5-hydroxymethylfurfural (HMF) and 2,5-furandicarboxylic acid (FDCA)³¹⁹

Substance	Structure	pKa	T m (K)	T b (K)	Water solubility (g L^−1)	Log P
a Solubility of FDCA in pure water.
HMF		12.8	302.15–308.15	388.15–390.15	Freely soluble	−0.778
FDCA		2.6 pKa1	>574.15	514.44	1 g L^−1 (292.15 K) ^a	−1.43
		3.55 pKa2

---

The solubility of FDCA in solvents (water, acetic, methanol, 1-butanol, isobutanol, MIBK, ethyl acetate, acetonitrile, 1,4-dioxane, tetrahydrofuran, 1,2-dimethoxyethane, and diethylene glycol dimethyl ether) at temperatures ranging from 308 K to 363 K is given in Fig. 11c. It can be seen that the solubility of FDCA decreases in the order of THF > DGDE > DX > DME > methanol > 1-butanol > isobutanol > acetic acid > water > MIBK > ethyl acetate > acetonitrile and further that FDCA solubility in each solvent increases with increasing temperature (Fig. 11).^320,321 Among these pure solvents, THF has the highest FDCA mole fraction solubility of 128.41 × 10⁻⁴ at 333.15 K. Some alcohols also have high affinity for FDCA because there are two hydrophilic carboxyl groups in FDCA molecules. When FDCA is in its non-ionized form, FDCA has low polarity,³²³ while methanol, 1-butanol and isobutanol are protic solvents, and FDCA molecules form hydrogen bonds with solvent molecules – to promote the dissolution of FDCA.³²²

FDCA is highly soluble in these solvents due to the presence of hydrophobic groups in the three alcohols and a polarity that is similar to each of the alcohols.³²⁴ Although acetic acid is also a protic solvent, the self-association between acetic acid molecules prevents the formation of hydrogen bonds resulting in low FDCA solubility.³¹⁶ The solubility of FDCA in water is limited due to the absence of hydrophobic groups in water molecules and due to the polarity of water differing greatly from that of FDCA.³²⁵ Acetonitrile, ethyl acetate and MIBK are all aprotic solvents, and they cannot form hydrogen bonds with FDCA, so the solubility of FDCA in these solvents is low.³²⁶ In short, FDCA may crystallize and precipitate in many solvents (water, acetonitrile), which is favorable for separation and purification of the product, but unfavorable for the catalyst surface because solid precipitation may block active sites or channels leading to catalyst deactivation.³²⁷ The separation of FDCA still needs to be explored further.

Temperature has a great effect on the solubility of FDCA in solvents. As mentioned above, the solubility in acetic acid at 363.15 K is about 6 times that at 313.15 K. Due to the sensitive temperature dependence of FDCA, the method of crystallization precipitation can be used for the separation and purification of FDCA. Massaro et al.³²⁸ achieved an increase in the purity of FDCA from 87% to 99% using eutectic crystallization (ca. 266 K).

Zhang et al.³²⁹ measured the solubility of FDCA in two binary mixtures (Fig. 11d). At a temperature of 313.15 K, the mole fraction solubility of FDCA in the two binary mixtures initially increased and then decreased with a rise in the mole fraction of water. For the water + acetonitrile mixture, the mole fraction solubility of FDCA reached its maximum when the mole fraction of water was approximately 0.6. For the water + acetic acid mixture, the molar fraction solubility of FDCA was greatest when the molar fraction of water was about 0.7. When water was added to the acetic acid system, the self-association of acetic acid shifted according to its pK_a value, and the mole fraction of FDCA increased. Under conditions of high-water mole fraction, the solubility of FDCA and solvent differs greatly, and the mole fraction of FDCA should decrease based on the Henderson–Hasselbalch equation. Maitra et al.³³⁰ found that when the solubility parameter of the solute was close to the solubility parameter of the solvent, the FDCA solubility was maximal. In a given binary mixture, the solubility parameter of FDCA may be close to the solubility parameter of the binary solvent, and the solubility can exhibit a maximum. The solubility of FDCA in organic solvent/water mixtures (Fig. 11) initially increases and subsequently declines as the fraction of water increases, indicating that FDCA can be purified through extraction.^320,321

The downstream separation and purification stage of FDCA production accounts for 30–40% of the overall production costs, presenting a major challenge for the efficient and cost-effective production of high-purity FDCA. Currently, acid–base neutralization is the most widely used separation technology for FDCA in industry. When an electrolyzed solution enters the neutralization reactor, H₂SO₄, 0.2–1 M, needs to be added to neutralize the solution until the pH drops to around 2, and then crystallization occurs and separation is possible. The waste liquid produced, which includes excess acid, salts, and other by-products, needs to be recycled through chemical operations, which greatly increases the cost of industrial processing. On the other hand, electrocatalytic oxidation of HMF under acidic conditions (pH < 2) can help to reduce the separation costs of FDCA in the later stages. However, one drawback in the use of acidic solutions in the electrocatalytic oxidation of HMF is that FDCA precipitation can affect catalytic activity. Additionally, about 20% of converted HMF is transformed into maleic acid, reducing the yield of FDCA.³⁵ Therefore, further in-depth research is needed on the design of separation and recovery systems to reduce costs. This could include the direct use of acidic electrolytes or temperature control to alter the solubility of FDCA, which would be beneficial for energy conservation and reducing environmental impact, ultimately leading to a competitive and economical FDCA production system.⁵¹

7. Techno-economic analyses

Based on the results discussed above, catalytic oxidation processes for HMF reactions can be evaluated using environmental indicators: feedstock sustainability (FS) and waste generation (WG); social indicators: innovation prospects (IP) and chemical safety (CS); economic indicators: product yield/selectivity (PYS) and process cost (PC) (Fig. 12). These indicators are normalized for comparison purposes, with 1 being the most favorable performance, 0 being the least favorable performance, and 0.5 being moderate performance.³³¹ The thermal catalytic process is the primary method for converting HMF into fine chemicals; this process typically operates at high temperatures and pressures, resulting in limited selectivity. Current state-of-the-art thermochemical purification techniques, including hydrodeoxygenation and gas phase purification, require large amounts of H₂ (e.g., H : O ratios of 50–500) and have relatively high carbon losses (15–20%).³³² Biocatalysts are usually highly selective to specific substrates and can reduce the production of byproducts. However, biocatalyst scalability is limited by factors such as enzyme stability, substrate class and product inhibition.³⁷ Krystof et al.³³³ found that DFF could be oxidized to FDCA (>99%) using in situ peracids as a biocatalyst. However, this method cannot be extended to HMF oxidation due to the specificity of biological enzymes. Photocatalysis technology can be used at room temperature and pressure with abundant solar energy as a driving force. However, its light-demanding properties may limit its application in certain environments or conditions.³³⁴ Photocatalysis, which is usually accompanied by severe side reactions such as deep oxidation, has a lower photocatalytic efficiency than thermal/electrocatalytic systems.³³⁵ The electrocatalytic oxidation process is usually carried out under mild conditions (such as room temperature and atmospheric pressure), which are conducive to reducing energy consumption and equipment costs. In addition, electrocatalysis technology can precisely control the reaction rate and product distribution by adjusting the current, applied potential and other parameters to make the reaction process flexible and controllable.¹⁵ Innovation prospects are assessed based on the number of articles published for each reaction pathway, which are identified by doing a keyword search for each response pathway using “Scopus”. Most studies have focused on alkaline environmental electrolysis (pathway I), but research studies investigating electrolysis under acidic conditions are increasing,^336,337 suggesting that the scientific relevance of acidic electrolysis of HMF is becoming more important than other methods. Electrocatalytic, thermocatalytic, photocatalytic and biocatalytic reaction systems for the oxidation of HMF were evaluated (Fig. 12). Thermocatalysis often involves organic solvents and noble metal oxidants, which are costly and produce a large amount of hazardous wastewater under high temperature and pressure.^146,338 Therefore, values of 0.9 and 0.8 were assigned for waste generation (WG) and process cost (PC), respectively. Electrocatalysis, photocatalysis and biocatalysis are relatively novel methods that can react under relatively mild and environmentally friendly conditions; values of 1.1 and 0.8 were assigned for innovation prospects (IP). However, photocatalysis requires high-level lighting conditions and has limited adaptability to solution systems, so a value of 0.6 was assigned for the process cost (PC).^25,334 Biocatalysts have high product selectivity (value of 0.9), but require intensive separation methods and long reaction times.^339,340 Electrocatalysis, as an environmentally friendly, low energy consumption and cheap catalyst for the green transformation of HMF, can achieve satisfactory yields, but has issues with energy efficiency.³⁴¹ Therefore, values of 0.6, 0.9 and 0.5 were assigned for waste generation (WG), product yield/selectivity (PYS) and process cost (PC), respectively.

Fig. 12 (a) Assessment of reaction routes for electrooxidation of HMF to FDCA according to normalized scores; (b) electrochemical industrial application prospects; (c) estimation of feedstock costs for producing FDCA over Ru₁–Co₃O₄ in 0.1 or 1.0 M KOH with 100 mM HMF; (d) levelized revenue of FDCA as a function of electricity cost and FDCA selling price. Reproduced from ref. 309 with permission from Wiley, copyright 2024. (e) Comparison of catalyst properties in different research studies. Reproduced from ref. 309 with permission from Wiley, copyright 2024. (■: Ru(III)–PEI@MWCNTs; ◧: NiVCo–LDH; ▲: RuNPs–Ni(OH)₂/NF; △: BiVO₄; ▼: NF-4; ◓: Mn_0.2NiS/GF; ◆: Ni₂P NPA/NF; ◨: Ni₃S₂/NF; ◒: Ni_xSe_y–NiFe LDH@NF; ●: Au₇/Pd₇; ★: t-NiCo–MOF; ☆: NiFe LDH; ▶: NiCoB_x; ◀: NCF; ◁: CuMn₂O₄; environmental indicators: feedstock sustainability (FS) and waste generation (WG); social indicators: innovation prospects (IP) and chemical safety (CS); economic indicators: product yield/selectivity (PYS) and process cost (PC).)

Six aspects of energy, carbon, technology, environment, catalyst and economy were used to analyze the industrial application prospects of electrocatalytic oxidation. Corresponding assignments are made according to the above assignment principles, and the results are shown in Fig. 12b. Compared with thermal catalysis, although it has the advantages of clean environmental technology and low-energy consumption, due to poor technical scalability and expensive raw material prices, the realization of industrial electrocatalysis to produce FDCA still needs further efforts. Compared with thermal catalysis, electrocatalytic oxidation has the advantages of mild reaction conditions, high catalytic efficiency, renewability and sustainability, but still faces many challenges in promoting practical industrial applications. Firstly, condensation polymerization reactions of HMF in high concentration alkali solutions limit the scalability of electrocatalysis technology.³⁴² The instability of electrolytic systems under high current densities (500 mA cm⁻²) is also an important issue that restricts large-scale application.²¹⁷ In recent years, researchers have also been looking for ways to address these technological issues. Zhou et al.³¹⁵ reported on a single-pass continuous flow reactor system that fed the substrate and alkaline solution separately, effectively shortening the substrate duration and thus reducing degradation during the conversion of high concentration HMF in high-alkali solutions. Although progress has been made in addressing the degradation of HMF, the use of highly concentrated alkaline electrolytes inevitably incurs high operational costs. Given that FDCA exists as a carboxylate in highly concentrated alkaline solutions, additional acids (such as H₂SO₄) are required to neutralize the electrolyte and subsequently isolate the target product.³⁴³ Previous studies have indicated that during the preparation process of FDCA, acid/base costs may constitute approximately 36% of the total production expenses for FDCA. This presents a considerable challenge for realizing industrial applications of electrocatalytic HMF conversion into FDCA in highly concentrated alkaline electrolytes.³²⁸ The degradation of HMF can be effectively inhibited in low-concentration alkali solutions (0.1 M KOH), leading to a notable reduction in carbon loss (around 40%) and material costs (by as much as 85%).³⁰⁹ In light of this, Lu et al.¹³⁸ proposed substituting high-concentration alkaline electrolytes (1 M KOH) for low-concentration bases (0.1 M KOH) to enhance the economic feasibility of HMFOR. However, at 0.1 M KOH, only 50% of the current density achievable with 1.0 M KOH is attained, posing a challenge for HMFOR based on low-concentration alkaline electrolytes.

Xia et al.³⁰⁹ reported on a single-atom ruthenium-modified cobalt oxide catalyst (Ru₁–Co₃O₄) employed for HMFOR within a flow reactor containing 100 mM HMF. The activity for FDCA production was found to be higher in 0.1 M KOH (yield: 75.8%, productivity: 141.4 mmol h⁻¹) compared with that in 1.0 M KOH (yield: 55.4%, productivity: 114.8 mmol h⁻¹). Due to the reduction of acid/alkali consumption and the increase of FDCA output, levelized feedstock costs were diminished by 65% when using a concentration of 0.1 M KOH (Fig. 11c). Furthermore, techno-economic analysis (TEA) demonstrated that changing the electrolyte from 1.0 M KOH to 0.1 M KOH resulted in a substantial reduction, by about 21%, in the minimum selling price (MSP) of FDCA. A lower E-factor (mass ratio of generated waste to target product) than that for 1.0 M KOH (185.4) was obtained (106.4), which further indicated that low-alkaline electrolytes were more conducive to the green and sustainable industrial production of FDCA. Fig. 12e illustrates the relationship between FE and current density based on recent advancements. While most research has focused on FE as a key metric, future investigations should examine industrial-grade flow cell systems that can operate continuously over extended periods of time at high current densities (500 mA cm⁻²).^171,223,344 Therefore, the electrode design of the cathode and anode must be optimized for these conditions, which may include enhancing the surface area of the electrode. Li et al.²²³ achieved FDCA productivity of up to 44.3 g h⁻¹ by expanding the Mn_0.2NiS/GF electrode area and assembling it in a continuous flow electrolytic cell; this provides a strategy for developing industrial-grade electrocatalysts that are compatible with high current densities. Regulation of the coordination structure of active sites is another promising approach. Chen et al.¹⁷¹ constructed Ni₃S₂/NF featuring stable and short Ni–S bonds by varying the water content in a solvothermal system. The production of gram-scale FDCA in MEA reactors at ultra-high current densities (1000 mA cm⁻²) demonstrates a practical application of the electrocatalytic oxidation of biomass-derived compounds. Alternative electrolyte systems with high oxidation stability can also allow for extended operation at increased current densities. The development of such systems would help to accelerate industrial applications.

Taking inspiration from the biological proton transfer mechanism, Chen et al.³⁴⁵ designed a ligand-modified (Ni(OH)₂–TPA) catalyst to address the slow kinetics of polyproton transfer during HMF oxidation. The uncoordinated carboxylic acid groups of TPA act as proton relay centers, accelerating charge transfer and providing a 16-fold increase in current density compared with that of pure Ni(OH)₂. The system operated stably for 240 h at 7500 mA, achieving a high productivity of 2.85 kg m⁻² h⁻¹. Ding et al.³⁴⁶ described a one-step ultrafast immersion method for preparing large area (NiCuO_x) catalysts. After 100 h of stable operation at 10 A current density and 200 mM HMF, 207.28 g FDCA was prepared with a purity of 99%. The work of Ding et al.³⁴⁶ provides new ideas for the development of scalable, energy-saving systems and bridges the gap between laboratory-scale catalysts and industrial needs.

Techno-economic assessments (TEAs) were developed to evaluate the prospective economic performance of industrial-scale 2,5-furandicarboxylic acid production. Major costs are associated with (i) raw materials, (ii) acid and alkali reagents, (iii) power and (iv) equipment. To calculate the actual process cost of electrocatalytic oxidation, the plant was assumed to operate for 8000 h per year (around 330 days) in continuous steady-state mode, with a service life of 20 years. Industrially relevant product concentrations (15 wt% HMF) were assumed to be fed into an electrolytic cell on the anode side. A yield of 95% FDCA and an industrial FDCA production of 33 000 tonnes per year was selected as the basis for evaluating the economic aspects of the process.³³⁶ Typical current density values reported³⁵ focused on 20–40 mA cm⁻², thus the current density was set at 30 mA cm⁻². The operating voltage in the electrocatalytic oxidation process was assumed to be 1.4 V vs. RHE, and the yield and FE of FDCA were assumed as 95% and 90%, respectively.³⁴⁷ According to the above parameter settings, an annual production of 10 000 tonnes of FDCA requires 11 500 tonnes of HMF, 24 400 tonnes of NaOH, and the separation section requires 32 300 m³ of concentrated H₂SO₄ solution. The annual electricity energy consumption is 27 000 MWh, almost all of which is used in the reaction section.

In addition, particularly focusing on the comparison of different methods and feedstock costs,³⁴⁸ the production methods for HMF and FDCA were considered and a summary of TEAs on chemicals related to FDCA production was compiled (Table 5). The FDCA product minimum selling price for HMF as a substrate is estimated to be $1445 per tonne via electrooxidation and is lower than that via thermal catalytic oxidation method (entries 1 and 2, Table 5) and this is related to the high FDCA selectivity achieved by the electrooxidation method.

Table 5 Techno-economic assessments (TEAs) made on chemicals related to 2,5-furandicarboxylic acid (FDCA) production

Entry	Feedstock	Process scale	Reactor type/reaction solvent	Product yield (%)	Product minimum selling price^a/unit (2023) ($)	Ref.
a All values were converted into 2023 US$ values; HMF: 5-hydroxymethylfurfural; FDCA: 2,5-furandicarboxylic acid; GVL: γ-valerolactone. (TR: thermal reactor; CSTR: continuously stirred tank reactor; PFR: plug flow reactor).
1	HMF	100 t d^−1	Electrolysis/aqueous-KOH	∼100	1445	336
2	HMF	10.5 t d^−1	TR/aqueous acetic acid	90	2100–3500	352
3	Fructose	300–500 t d^−1	CSTR/aqueous–organic phases	70–87	470–1890	355 and 356
4	Glucose	140 t d^−1	PFR/MIBK	94	2000	353
5	Cellulose	900 tCel d^−1	CSTR/aqueous-H2SO4	—	1532	8
6	Seaweed	5 t d^−1	CSTR/aqueous-H2SO4	10	2890	357
7	Wood chips	2000 t d^−1	CSTR/GVL/THF	40	1300–2000	358–360

---

The production of FDCA from biomass-related feedstocks, such as glucose, fructose, and cellulose through electrocatalytic oxidation has been highlighted as being economically attractive due to its prospects for eliminating intermediate pH regulation and separation steps.³⁴⁹ However, TEAs of electrooxidation production of FDCA from biomass have not been widely made; however, TEAs have been performed for the production of HMF from biomass-related feedstocks through thermal catalytic methods. The TPA cost of FDCA could fall to $470–1890 per tonne from fructose, $2000 per tonne from glucose, and $1532 per tonne from cellulose through thermal catalytic oxidation (entries 3–5, Table 5). While the TPA cost of FDCA could be expected to increase to $2890 per tonne from seaweed and $1300–$2000 per tonne from wood chips through thermal catalytic oxidation at lower yields (10–40%). It could be suggested that FDCA yields and feedstock costs are key factors that influence the profitability of production plants.^350,351 The cost of HMF contributes greatly (∼77%) to the total costs in the electrochemical production of FDCA, compared with the thermochemical pathway (39%).³⁵² This difference is attributed to other process-related costs, such as capital and operating (reactor, separation, energy, operation, and maintenance), which are lower for electrochemical production in comparison with thermochemical production.³⁵³ An acidic solution electrolyte is required for efficient electrocatalytic systems in the one-step synthesis of FDCA from glucose or glucose-based feedstock, to reduce the cost of pH regulation and FDCA separation.^328,354 There remains limited reports in the literature on TEAs such that further studies on techno-economic assessments of FDCA production from biomass-related compounds including lignocellulosic biomass are needed.

Overall, the industrial implementation of electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) faces several critical challenges that demand systematic optimization across areas of reaction engineering, catalytic materials, and device design:

7.1. Mass transfer and concentration limitations in reaction systems

Mass transfer and concentration bottlenecks of the HMF to FDCA reaction system hinder industrial application. Biomass-derived HMF solutions typically have low concentrations (<50 mM), which limit mass transfer due to the feedstock. Biomass pretreatment processes tend to have insufficient enrichment efficiency of target products, which leads to faradaic efficiency attenuation and increases energy consumption. Mass transfer resistance makes the supply of active species at the reaction interface unbalanced and slows oxidation kinetics.

7.2. Selectivity control in multistep oxidation pathways

Selective conversion of HMF to FDCA remains challenging due to competing side reactions involving active intermediates. The electrocatalytic process requires multiple electron transfer steps, during which intermediates (such as HMFCA and FFCA) undergo side reactions due to different reactivities and thermodynamic stabilities, such as C–C bond cleavage that affords small-molecule carboxylic acids (such as acetic acid and formic acid). Inhibition of side reactions can be challenging for electrocatalysts at high operating voltages, where strong adsorption leads to excessive oxidation and weak adsorption leads to desorption and inactive sites. Under industrial conditions, selectivity can be improved through a number of strategies such as optimizing electrolyte composition (e.g., phosphate buffering systems to stabilize key intermediates), designing catalyst surfaces with atomically dispersed active sites (e.g., monoatomic catalysts or coordination isolation structures) that lower non-specific adsorption, and designing microchannel flow batteries to enhance liquid mass transfer and shorten the residence time of intermediates.

7.3. Multiscale degradation mechanisms of catalyst stability

Industrial application costs are increased due to catalyst instability that results from multiscale failure mechanisms. Transition-metal-based catalysts (e.g., Ni, Cu) suffer from cation dissolution in acidic or alkaline electrolytes, leading to active site loss and potential contamination of FDCA products. Even noble-metal catalysts (e.g., Pd, Pt) experience surface reconstruction or nanoparticle agglomeration during long-term operation, reducing the active specific surface area. Recent advances include stabilizing catalysts via core–shell structures, oxide-derived frameworks, and solid electrolyte interfaces. However, industrial requirements for catalyst longevity (e.g., stable operation >1000 h with <5% activity loss) necessitate further development of corrosion-resistant strategies in atomic coordination, interfacial electron structure, and nanograin boundary engineering levels. Maintenance schedules (e.g. changing electrodes after a given number of hours or parallel system switchover) can help to lower the severity of catalyst instability.³⁶¹

7.4. Scalability challenges of nanostructured electrodes

While nanostructured electrodes (e.g., porous membranes, 3D frameworks) enhance accessibility to active sites, their industrial scalability is hindered by manufacturing complexity, cost, and mechanical instability. For example, controlled synthesis of high-precision nanostructures is incompatible with large-scale production, and binder-free electrodes or electrodeposited coatings may degrade under high current densities (>1000 mA cm⁻²) due to structural collapse or interfacial stress-induced delamination.

Addressing these challenges will require the interdisciplinary integration of catalysis, electrochemical engineering, and materials science fields to precisely tune reaction pathways on the molecular level along with optimization of mass and energy transfer at the device level. Such an integration of research disciplines will enable the translation of laboratory prototypes for electrocatalytic HMF oxidation to viable industrial processes.

8. Conclusions and future outlook

The effective conversion of HMF into FDCA via electrooxidation is important not only for sustainable chemical production but also to satisfy global needs for environmentally friendly plastics. Considering the increasing emphasis on electrooxidation technology for the production of FDCA from HMF in the field of green and sustainable chemistry, this work has analyzed the primary factors influencing the performance of electrocatalytic reactions and systems as well as methods for FDCA separation and purification. The main conclusions and perspectives of this work are as follows:

8.1. Optimization of electrocatalysts

Of the precious metal, transition metal and non-metallic catalysts proposed for the electrocatalytic oxidation of HMF, Ni-based catalysts show the greatest application prospects. However, technical issues related to few active sites and lack of stability still restrict the use of Ni-based electrocatalytic systems. Through defect and vacancy construction, crystal structure regulation, doping with heteroatoms (N, S, P), and control of the catalyst morphology and particle size, sufficient electron transfer can be achieved, active site exposure and specific surface area can be increased, and catalytic performance can be improved. On the other hand, the preparation of electrocatalysts for continuous and stable operation at high current densities (≥500 mA cm⁻²) will require innovative electrocatalytic reactor designs.

8.2. Design of electrolytic cells

From an industrial application perspective, unlike common single cells, H cells and flow cells, the employment of zero-gap membrane electrode assembly (MEA) electrolytic cells can reduce interface contact resistance, promote ion transport and the start-up response, and provide a low and stable voltage for long periods of time. However, poor corrosion resistance and the high cost of porous transport layers (PTLs) and bipolar plates (BPPs) under acidic conditions limit their industrial application. In addition, MEA is not widely used at present, and there are still many basic issues to be studied to fully understand the interaction of components and parameters (flow rate and liquid supply mode) that influence the performance of electrolytic cells.

8.3. Applicability of paired reactions

The coupling of HMF electrooxidation with other reduction reactions, such as hydrogen evolution, carbon dioxide reduction and organic catalytic hydrogenation, shows the possibility of improving energy efficiency in multi-directional applications. However, it should be noted that the voltage/current density and pH required for the anode and cathode reactions are not the same and cannot be controlled independently in a two-electrode system. To address this issue, it will be necessary to develop bifunctional electrocatalysts that promote reduction reactions and match the parameters of the HMF oxidation rate.

8.4. Electrolyte regulation

The electrocatalytic oxidation of HMF to FDCA in highly alkaline electrolytes (concentrations ≥1 M, pH ≥ 13) has been well studied with commonly reached conclusions being that HMF is not stable in strong alkaline electrolytes (≥1 M) for which condensation polymerization reactions readily occur to form humins and humic substances, thereby reducing FE.³⁶² To address this issue, several strategies have been proposed, such as replacing the alkaline electrolyte with an acidic one, reducing the concentration of the alkaline electrolyte and coupling it with a flow tank reaction system or implementing a feed separation strategy for the substrate and alkaline solution. This combination not only resolves the issue of HMF degradation, but also enhances mass transfer. More attention should be paid to organic electrolytes that have a wide electrooxidation window, controllable product selectivity and strong solvation characteristics.

8.5. Product separation

Currently, FDCA separation primarily relies on regulating the pH level of the electrolyte to exploit solubility differences. The acid–base cost used to regulate pH may account for a significant portion (36.4%) of the total production expense of FDCA, which is not conducive to cost savings and sustainable development. Most studies on the electrooxidation of HMF usually focus on the basic catalyst, while overlooking acidic/neutral catalytic systems. In subsequent studies, the design of high-performance catalysts (such as Ir and Ru) suitable for acidic/neutral systems should be considered, such that the spontaneous precipitation of the FDCA product can be achieved through temperature control (393 K to 413 K) to reduce separation costs.

8.6. Techno-economic assessments

To realize the electrocatalytic oxidation of HMF to FDCA on an industrial scale, technical and economic assessments are required to identify process hurdles and bottlenecks. Electrocatalytic oxidation of HMF to produce FDCA has lower energy, operation and maintenance costs than thermal catalysis. However, the high cost (∼77%) of the HMF raw material prevents further industrial application of the electrocatalytic oxidation of HMF to produce FDCA. Direct electrocatalytic oxidation of FDCA from biomass-related feedstocks such as glucose, fructose and cellulose could eliminate intermediate pH regulation and separation steps, resulting in favorable economics. In future studies, the design of thermoelectric equipment should be considered to achieve the direct oxidation of FDCA from biomass feedstock.

Abbreviations

ACT	4-Acetamido 2,2,6,6-tetramethylpiperidinyl-1-oxide
BHMF	2,5-Bis(hydroxymethyl) furan
CO2RR	Carbon dioxide reduction reaction
COFs	Covalent organic frameworks
CSTR	Continuous stirred tank reactor
CV	Cyclic voltammetry
DFF	2,5-Diformylfuran
DGDE	Diethylene glycol dimethyl ether
DHMF	2,5-Dihydroxymethylfuran
DME	1,2-Dimethoxyethane
DX	1,4-Dioxane
ECH	Electrocatalytic hydrogenation
FA	Furoic acid
FDCA	2,5-Furandicarboxylic acid
FE	Faradaic efficiency
FFCA	5-Formyl-2-furancarboxylic acid
FUR	Furfural
GVL	γ-Valerolactone
HEAs	High-entropy alloys
HEMF	2-Hydroxymethyl-5-(ethanolamine methyl) furan
HER	Hydrogen evolution reaction
HMF	5-Hydroxymethylfurfural
HMFCA	5-Hydroxymethyl-2-furancarboxylic acid
HMFOR	5-Hydroxymethylfurfural oxidation reaction
HOR	Hydrogen oxidation reaction
HPLC	High performance liquid chromatography
LDH	Layered double hydroxides
LSV	Linear sweep voltammetry
MA	Maleic anhydride
MEA	Membrane electrode assembly
MOFs	Metal organic frameworks
NPs	Nanoparticles
OER	Oxygen evolution reaction
PEF	Polyethylene-2,5-furan dicarboxylic acid
PFR	Plug flow reactor
PTL	Porous transport layer
SFG	Sum frequency generation
TEA	Techno-economic assessment
CS	Chemical safety
FS	Feedstock sustainability
IP	Innovation potential
PC	Process cost
PYS	Product yield/selectivity
WG	Waste generation
TEMPO	2,2,6,6-Tetramethylpiperidinyl-1-oxide
THF	Tetrahydrofuran
TPA	Terephthalic acid
TR	Thermal reactor

Author contributions

Bingkun Chen: Writing – original draft preparation, data curation, conceptualization, methodology; Qidong Hou: Writing – original draft preparation, data curation; Richard Lee Smith Jr: Writing – reviewing and editing, methodology, supervision; Xinhua Qi: Writing – reviewing and editing, conceptualization, methodology, supervision; Haixin Guo: Writing – original draft preparation, writing – reviewing and editing, conceptualization, methodology, funding acquisition.

Data availability

All data generated or analyzed during this study are included in this published article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to the Elite Youth Program of the Chinese Academy of Agricultural Sciences for funding (to Haixin Guo).

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